This Probability distribution of testing time of the software package is a problem waiting to be solved quickly. We transfer it into settling a series of problems which related to continuous-time Markov chains. Firstly, we construct a Q-matrix corresponding with the Markov chain X(t). Secondly, we solve the Kolmogorov forward equation corresponding with the Q-matrix and obtain the transition function Pij (t) of the Markov chain X(t). Finally, we get the probability distribution, expectation and variance of testing time of the Software.