There are some requirements about what need to be included in the letter , such as why, when, where to live and who will support travel, expenses, health insurance etc. Check that website.
In my understanding, the best reason is you miss your mother and so does she.
You need documents to convince the visa officer that your mother is financially able to travel and would not stay illegally after.
e.g. her stable income in China, a house or car, her career or social status, other children, a letter from her boss......
hope this helps.