The premise of designing a motor in a scientific way is to derive the factors that influence the performance of the motor in theory. In an attempt to derive the factors that influence the electromagnetic torque and efficiency of a permanent magnetism Brushless Direct Current (BLDC) motor and verify the factors through virtual stimulation, this paper derives the formulas that associate designing variables with electromagnetic torque and efficiency based on the electro-mechanic viscous damping theory and other basic electromagnetism theories. It is concluded from the formulas that the electromagnetic torque and efficiency of the motor can be presented with a multivariate function. The determinate factors are designing parameters including rated voltage, coil half-turn length factor, and the depth, number of poles, and air gap of the permanent magnet. The tendency of designed variables associated with electromagnetic torque and efficiency is shown through mathematic analysis. In order to further verify the theory, the RMxprt module of ANSYS Electronics Desktop software is used to stimulate and analyze influencing factors of the motor performance, and the results conform to the theory. The tendency will provide references to permanent magnetic BLDC motor researchers in their selection of designing variables and adjustment of the variables.