What are the requirements for being admitted to Peking University Institute of Remote Sensing? Need an article? Need a recommendation? When is the appropriate time to contact the tutor?

There is nothing to say. As long as you find a good tutor and review the subjects of the exam carefully, there will be no problem. As for SCI and EI, they are the assessment criteria for doctoral graduation. It's good to have them, but it doesn't matter if they don't. Mentors should contact early, and it is best to ask acquaintances for help, otherwise the chances of success are very small.