Applying Changli, Baodi, and Beijing geomagnetic vertical component data, the correlitivity with Hongshan was described. The results show that at all of three stations occurred abnormal variation before Tangshan earthquake. Abnormal variation at Changli was the most obvious, abnormal period lasted for a couple of months, abnormal amplitud was about-10 nt. Based on the results of the references(1,2) and this paper, the mechanism of the geomagnetic abnormal variations was discussed. The process, preseismic instability of inclusion-porous fluid diffusion-dilatation and hardening of inclusion, probably was the reason for the above geomagnetic abnormal variation.