The transmission ratio along the radian direction normal to the joints was studied in the Universal Distinct Element Code (UDEC). The variation of the transmission ratio with the ratio of joint spacing to wavelength was generalized into a general curve, which was determined by two critical points. The relationship between the two critical points and the affecting factors, quantity of joints and the normalized normal stiffness of joints, were obtained. A prediction model of the transmission ratio in the radian direction normal to the joints was proposed. The proposed model was applied to a field explosion test. The estimated values of the peak particle velocity from the prediction model were compared with the field records. The comparisons showed that the prediction model of the transmission ratio in the direction normal to the joints in the process of 2-D compressional wave propagation through multiple parallel joints is reli- able. The transmission ratio along the radii direction normal to the joints was studied in the universal distinct Element Code (UDEC). The variation of the transmission ratio with the ratio of joint spacing to wavelength was generalized into a general curve, which was determined by two critical The relationship between the two critical points and the affecting factors, quantity of joints and the normalized normal stiffness of joints, were obtained. A prediction model of the joints was proposed. The proposed model was applied to a field explosion test. The estimated values ​​of the peak particle velocity from the prediction model were compared with the field records. The comparisons showed the prediction model of the transmission ratio in the direction normal to the joints in the process of 2- D compressional wave propagation through multiple parallel joints is reli- able.