Abstract: To investigative the relationship between the pressure angle and other design parameters of the whole cycloidal gear. Deals with the derivation of the pressure angle formula for the whole cycloidal gear in theory, draws the curve of the pressure angle by selecting other design parameters such as modulus, the number of teeth, radius, and finds the relationship between them. The curve of pressure angle has nothing to do with modulus but the number of teeth if the two rolling circle have the same radius. The relationship between the pressure angle and other design parameters of the whole cycloidal gear is found out, and proves the possibility that the whole cycloidal gear can be used in an internal parallel move gear mechanism. It also provides a theoretical fundament for designing incorrectly an internal parallel move whole cycloidal gear.