when connect with pppoe on my ppp server then ping same server , ping is 20ms ...
(connection between my computer and ppp server (mikrotik 2.9.51) is over ethernet)

after disconnet from ppp my ping is 1-3 ms...

how to solve this problem ?

ppp server cpu all time is abaut 50%

I setup a pppoe connection from my computer to mikrotik.

Ping = 1ms with or without pppoe.

I doubt you will get much help while using version 2. Could you update to version 4?

First, my version is 2.9.51.

Today , upgraded to 3.28 , and problem is same.

In this momment, connected ppp user is about 400 and traffic is about 50mbps/14mbps.

Cpu usage is about 30%.

Maybe problem is ethernet card ?

Yes, I realize you were using version 2... that's why I said "version 2".

I don't see how your ethernet card could cause high ping with pppoe, but low ping without pppoe.