The ray transmission characteristics of off-axis prism-pair are analyzed using geometrical optics method at different offset. The numerical model of off-axis prism-pair is built by ray tracing. The numerical relationship between the offset d and hypotenuse L of the off-axis prism-pair is obtained. The transmission direction, the number of ray transmission and the distribution of ray in the off-axis prism-pair are discussed at three different cases.The results show that the rays transmitting in the off-axis prism-pair have equal distance space distribution pattern when d=L/2n (n=1,2,3…)so it is much more benefit to obtain unform gain for each pass of rays and to realize multi-pass and amplification of slab-shaped gain medium.