I am confusing now is during the timer0 count period, if the key is pressed, the program will jump to keyboard ISR,after ISR, the program will start from where? the beginning of keypressed subroutine
or going on counting timer?

ISR shall return to where it branches off. This is how an interrupt works.

I don't think you should worry about keyboard debounce, which should have been taken care at the keyboard end. Your problem may well reside on the structure of your code. You should use EX0 (Clock) to reload timer0 for 50ms inside your EX0's ISR. The enabling of timer0 interrupt can take place in your initialization routine. This timer0 interrupt shall not occur for as long as you are receiving serial data. When timer0 interrupt happens, you'll have your receive buffer ready. You set a flag and have your background program start processing the data in the buffer.

Hope this helps.