To design a better output performance radial piston motor with inner curve, a new-type inner curve which is named modified heart-shaped curve is presented. The curve is simple, continuous, closed and higher-order differentiable. Using this curve as an inner curve of radial piston motor, the internal impact problem is solved. The influence of the piston’s number and periodicity to the phase position are analyzed. The result shows that the motor’s pulsation rate depends on the equivalent number of piston. A general formula for the pulsation rate of the motor with the modified heart-shaped curve as an inner curve is derived, and the formula shows that the motor’s pulsation rate decreases with the increase of the equivalent piston number. At last, the conclusions are certified by simulation.