It aims to numerically study the transmission ratio in the process of 2-D compressional wave propagation through multiple parallel joints in rock masses along the special radian direction normal to the joints, using the Universal Distinct Element Code (UDEC). The effects of three possible factors influencing the transmission ratio have been investigated. These influencing factors are the ratio of joint spacing to wavelength, the number of the joints, and the normalized normal stiffness of the joints. For any given level of the normalized normal stiffness of the joints and any given number of joints, the variation of the transmission ratio with the ratio of joint spacing to wavelength was generalized into a characteristic curve determined by two critical points. The relationships between the two critical points and the influencing factors were obtained as a model for estimating the transmission ratio along the special radian direction normal to the joints.