回複：Parents visit

employer letter should be wrote by your employer not you, in the letter just state your visa status, what's your job titil and when you start work in this place, also how much you earn a year, if your job is permanent, put it in the letter too.

Also you need to write a invitation letter to the US consulate in China state why you want to invite your family to US, their name, DOB, and state you will provide all the expense for them while they stay in US.