I am in the H-1B visa, and it is near my 6 year H-1B limit. I want to change my job and I have an Employment Authorization Document (EAD) currently, which I received from USCIS after my I-485 application. 

Since my I-485 has been pending for more than one year, can I use my EAD after my job change based on the AC-21 rule? or I have to file a H-1B visa transfer application to USCIS with my new employer?  

You can use your Employment Authorization Document (EAD) for working with the new employer. But the better way is that you can also file a new H-1B visa transfer application with my new employer. 

This is possible because according to the AC-21 rule, you are eligible for H-1B extension even after the 6 year H-1B visa limit, if your form I-485 application is pending for more than 180 days. Therefore, using the EAD to work for the new employer is not the only option.