A RV cycloid planetary reducer used in the excavator chassis is studied. A contact model based on impulse function is proposed to define the constraint between the contact parts during the working condition. The contact parameters in the contact model includes: Contact stiffness, index of force, damped coefficient and depth of penetration. Among them the unique undetermined one is contact stiffness, which is studied under two working conditions. These conditions are Convex-Convex Contact (pin-top of cycloid) and Convex-Concave Contact (pin-dedendum of cycloid). Analysis of the contact parameters indicates that the contact stiffness is affected by the contact thickness. The performance of the reducer part material must satisfy these parameters. Then a dynamic model of the RV cycloid planetary reducer is built, in which the contact force model is utilized to realize the constraint of pin-cycloid wheel, planetary wheel-sun wheel. The simulate results of idle load and full load indicate that the deviation of output draft angular velocity between simulation and theoretical is 0.83%. The work envelope of the center of the cycloid mass is a circle, and the circle radius is eccentric distance of the two cycloids. The simulation of contact moment between the pins and cycloid reveals only six contacts exist at any instant, and the maximum moment appears at the middle contact pair. This contact constraint model can be used to direct the machining of the wheels and the process of reducer assembly.