Abstract: The crystal basement velocities and time-terms in North China region are imaged by a tomographic backprojection using Pg waves from 28 pro filles. Quantitative analyses of the algorithms are given by synthetic data of the digital model. The results from the test show that our method is quite stable and reliable. The LSQR algorithm is used for solving overdetermined equations. The zero elements of the large sparse matrix are not stored and calculated. Thus the store amount is reduced to one percent. The speed of calculation is raised greatly. The minimum and maximum velocities are 5.9km/s and 6.35km/s respectively. The maximum time-term is 2.0 second.