Writing Passage - titles are usually capitalized

Since Spring Festival (the Spring Festival is an event, so it is captialized), I haven't writing written (writing is a present tense verb. That means the action is happening at that moment. Since you used the word haven't, you are referring to the past, meaning you must use the past tense version of writing which is written) passage.

The sentence "Since Spring Festival, I haven't written a passage." is not a wrong sentence, but it is not the casual way of speaking. We normally say, "I haven't written a passage since Spring Festival."

I found that it is a better way to improve English (degrace? I'm not sure how you want to use this word. I have never seen it in this context before) through reading other people's passages. (because people own their passages, people must be corrected to people's to show possession of the passages, and passages must have the s at the end because there is more than one passage for all the different people. They don't all share one passage, there are more than one.)

Now, this sentence is not technically incorrect either, but it can sound more fluent slightly altered. "I found that it is a better way to improve English through reading other people's passages." can be changed to "I found that reading other people's passages is the best way to learn English." In the original sentence, you used it, which is referring to an object we, as readers, don't know. To help fix this, put what it is referring to in its place--which in this case is reading other people's passages. I also changed is a better into best because if you say better, you need to be comparing things. Like apples are better than oranges. You can't just say apples are better because we don't know what you are comparing the apples to. Alternatively, you can say apples are best because we can assume you like apples the most out of anything you compare it to.

because own passage is still keep myself word. others use the different way to express their things. (I'm not exactly sure how to correct this sentence, but I rewrote my interpretation of what you were trying to say.)

I find it helpful to read other people's passages because others express themselves differently in their passages than how I express myself in my own passages.

I think it is very useful for me. (good as is)

I hope I helped! :)