Cambodian Citizen first time apply Chinese visa in KTM

Chai, if you have valid entry permit to Nepal, it’s possible for you to apply for it from there. You can apply for it on your own, but it's advised to have an agency to apply for you. Firstly, prepare the required documents including your passport and its copy, the application form with a passport-style photo attached, your valid entry permit to Nepal, proof of transportation and accommodation arrangement, proof of your financial support and your personal statement.
Next, go to the embassy of PRC to submit the application.

Thanks Golden for your information. Do u know someone or travel agency which can help me to apply it? Please give me the contact detail. Do applicant need to show up in the embassy or interview ?

I have no idea of relevant agency in KTM, sorry.
If you are required to show up for an interview, you need to go there in person and accept the face-to-face interview.