+2
When I was a kid, I was obsessed with computer games, but I didn't have much money to play computer games. So I told my mother that I needed RMB20 to purchase a new book. Actually I used the money to play computer games. Once my mother wanted to see the book I bought, I didn't expect her request and had to tell the truth. My mother was furious and punished me severely. Since then, I never treated my mother out of money.
Comments  
Hi Yue Ming, it's a good writing!
I think there's nothing much to correct, just some suggestions:

-When I was a kid, I was obsessed with computer games, but I didn't have much money to play computer games. (Replacing computer games with them is better, because you already wrote computer games in the previous phrase)
-So I told my mother that I needed RMB20 to purchase a new book. (This is good, you can also say: I needed RMB20 to buy a new book/I needed RMB20 for a new book)
-Actually, I used the money to play computer games. (It's fine to add a comma after actually. This one is good too, and you can also say: I spent the money playing computer games)
-Once my mother wanted to see the book I bought, I didn't expect her request and had to tell the truth. (This is fine, using When instead of Once is okay. I think I bought isn't necessary here, as we know that it's the book you said you would buy)
-My mother was/got furious and punished me severely.
-Since then, I have never treated my mother out of money (again). (I don't quite understand what you mean with treated my mother out of money) Emotion: smile

You'd better ask for more opinions,
GN
Thank you so much.
"I have never treated my mother out of money"
Sorry, here I made a mistake. It should be "I have never cheated my mother out of money"
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Oh, I see. So that's the point Emotion: smile Would you like to addagain here? I have never cheated my mother out of money again.
Because you already did the cheating once, so you wouldn't do it again. It can be without again if you didn't do the cheating yet.
GN