This paper discussed the solution of the Bloch equations of a two-level atom resonantly interacting with laser pulse trains in detail. In terms of the solution, we calculate the radiation spectrum of the two-level atom excited by the laser pulse trains and obtain the following conclusions: (1) for the atoms satisfying the condition K1/K = 1 (such as gas atoms broadened by spontaneous emission), the laser pulse trains are almost equivalent to a CW laser; (2) for the atoms K1/K《1, saying, some solid atoms lik...