As the phd course will last for four years. Do i have to pay for the tuition fee for these whole 4 years, or pay it yearly? If my family save money for my tuition fees yearly and pay them year by year, can i get the visa? How can i convince that i do have the capability to fanance my self but not at one time?

You pay yearly. Nobody will ask you to submit entire four years' fee. Instead, you can arrange to pay your fee on semester basis. For a visa, you need to pay a semester's fee in advance + show proof you can pay for at least one year.

Thanks Raj, I was told by the advisor just now that maybe the embassy will ask for my parent's pay slip, but will they suspect that my parents will spend up all they earn rather than saving it for my study. i still don't think i can convince them. Do you know if there are any cases in the same situation? how did them worked out if so?

your parents account has to show enough money for 4 years, my parents account only showed enough money for a year but i got a US visa, but i think i took more than one account statement, not sure though.

You may be granted a visa if your parent's account has enough money for at least one year and you have paid 1st semester's fee.

Also, if your parents can prove that they can pay for your studies, the embassy will not care whether they will spend all their earnings. There are several parents who spend their life's earning to educate their children and immigration officers know this. (Remember the one year amount as a base).

Thank you very much Raj. Yes, i can pay for the first year, but does this mean i can get the one-year visa, and i have to extend it yearly by showing them i have got the enough money for a futher year?

Also thank you so much imm, i don't understand what you meant by'taking more than one account'?

You will not get a student visa for more than three years for a phd. You can extend it later if you dont finish in three years. Extending a visa is a lot easier than getting one for the first time.