In order to calculate and evaluate the performance of the motors accurately and quickly, an analytical approach for determining noload airgap flux density of permanent magnet motors considering multi factors based on the I type rotor structure was proposed. A prototype of 48slot 8pole permanent magnet motors was simulated, and the good agreement between the calculated data and the simulated data verified the validity of the proposed analytical approach. The influence of parameters such as permanent magnet size, height of isolation bridge and pole arc coefficient on air gap flux density was analyzed, which provided a way to optimize the motors scheme.