My son is studying in China, Tianjin, he will be there for another 4 years. can i get multiple visa.

Pillai, you can apply for an S1 type which can allow you to get a residence permit and the residence permit can allow you to enter and exit PRC for multiple times. The required documents contain your passport and its copy, the application form, a passport-style photo, an invitation letter from your son, and copies of his passport and residence permit, as well as the relationship certificate.

As for HK transit entry permit, you need to provide copy of your passport, the application form affixed with a passport-style photo, proof of your financial support, transportation arrangement and the onward ticket to the next destination. After, submit the application to the embassy of PRC to apply for the transit entry permit.