In this paper, we first introduce two basic methods for calculating the positron wave function: the finite differential method and the plane wave method (PW). Then taking Si as an example, we calculate the positron wave function from the wave function, and get the positron density distribution in silicon. Then we obtain the bulk positron lifetime from the positron density. The calculation result agrees with our latest experimental result of 220 ps. At last, we discuss the respective advantages and disadvantages of the two methods.