Bad idea. Get a friendly supervisor to write it, and when submitting it, write a cover letter to immigration officer tell them NOT to mention immigration if they check with the employer because you don't want to lose your job BEFORE getting your visa.
Just to help your English. Please do not use it for your visa application
An example letter:
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May 15, 2004
To Whom it may concern:
This letter will serve to confirm that Ms. Carol Feng is employed with ... (?位). Ms.Carol Feng has been employed by ... (?位) since January 1, 1998 on a full time basis. Ms. Carol Feng is a ... (职位) and her gross income is Chinese $100,000 per annum.
Should you require further information, please do not hesitate to contact me at 010-xxxxxxxx (电话).
Yours sincerely,
XXXXXX
Specialist
Compensation and Benefit
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Human Resources