The purpose of this paper is to outline a photoelastic experiment with a model for the tectonic stress field of the Tangshan region under a compression in N60°E direction. In the same experiment, the stress release of the strong earthquake was also simulated by using the method of hole-drilling. The results are as follows:
1. When the Tangshan region was undergoing a compression in N60°E direction, the maximum shear stress was concentrated at the Tangshan-Fengnan district, Yejituo village, Luanxian county and Lulong county. These stress concentration regions were mostly corresponding to the epicentral areas of the Tangshan earthquake(M=7.8), the Luanxian earthquake(M=7.1) and their strong aftershocks. It has been recognized that the occurrences of the Tangshan earthquake and its aftershocks were due to the compressive stress field in nearly EW direction.
2. Though the stress was released during the Tangshan earthquake (M=7.8), it still increased in those areas when the background value of stress was high before the earthquake event. This shows that a readjustment of the stress field was caused by the Tangshan earthquake, thus intensifying the dynamical conditions for the occurrences of the strong after-shocks.