It's not really awful. Your basics are OK and your vocabulary choice is fine, but make sure to remember that these are people. As Ruby suggest, you need to vary the structures. What does she mean? Let's look at one of your paragraphs.
One point I would love people to remember about graphs in IELTS is that there are usually PEOPLE behind the lines and numbers and tables. You wrote:
Sweden and USA had the similar tendency. In 1940, the ratio in Sweden and USA are larger than Japan, 7% and 9% respectively. The increasing speed in Sweden was faster than USA and overpassed them in 2000. After that point onwards, both two nations continued surging, and will be surpassed by Japan around 2035. Finally, Sweden and USA would stand at 25& and 23% correspondingly.
What is this paragraph about? Yes, you’ve described a few lines and said if they go up or down. That's OK. It's like the cartoon here:
But behind the graph, there are people. Nowhere in this paragraph have you mentioned people: Elderly people, old people, the number of young people, the number of old people, the working population, how many young people will have to take care of the older people, etc.
This is called a ‘mechanical’ description (Have a look at the official descriptors for IELTS Task 2 Writing (public version), under Task Fulfillment.)
You do mention people in your final paragraph, but it’s important to remember throughout.
Word Choice/Word Form/Usage
- The line graph portrays the percentage of the elderly who beyond 65 in three countries → The line graph portrays the percentage of the elderly who are over/ are aged over/ 65 in three countries
- Each line had an increasing tendency over the 1 century. → Each line had an increasing tendency over the century. NO!
- The number of older people as a percentage of the population will increase sharply in all three countries.
- Specifically, there were only 5% old individuals live in Japan since 1940. → Only 5% of Japanese people in 1940 were over 65.
- Then, the rate had decreased to 3% gradually by 1960 → Then, the proportion/percentage of people over 65 years decreased gradually to 3% by 1960 [Keep the adverb near the verb, and it’s not a rate – it’s a percentage or a proportion.]
- In 1940, the ratio in Sweden and USA are larger than Japan, 7% and 9% respectively → In 1940, the ratio in Sweden and USA are larger than Japan, with/at 7% and 9% respectively.
The increasing speed in Sweden was faster than USA and overpassed them in 2000. After that point onwards, both two nations continued surging
This needs a complete rewrite:
The number/percentage/proportion of older people in Sweden grew more quickly than in the USA, where the proportion stayed steady at 14% of the population. From 1995 onwards, the percentage of Sweden’s elderly began to rise sharply and is expected to reach 1 in 5 of the population by 2015.
The proportion of people over 65 in America is predicted to rise rapidly from 15% in 2020 to almost one quarter of the population in 2040.
However, Japan will have the sharpest rise in the proportion of old people. It is estimated that the percentage will jump from just 10% of the population in 2030 to almost 25% in 2040. In other words, there will be only three younger people for every Japanese pensioner over 65 in 2040.
So your basics are OK and your vocabulary choice is fine, but make sure to remember that there are people, not just lines!