In this paper，a contour surface equation has been developed for the cylindrical cam with oscillatig roller follower by utillizing enveloping theorm of differential geometry. Based on it，some essential problems，such as the condition of cylindrical cam's undercutting，the contact zone and actual contact curves between the roller and cam，etc.，have been discussed in detail，and the formula of perssure angle as well as the minimum contact height of rollers has been derived too. All of these make the analytical design method for the cam more perfect.