Abstract: The demagnetization field during the starting process of line-start permanent magnet synchronous motors（LSPMSM） may cause the irreversible demagnetization of permanent magnet.For the further study of the causes of the demagnetization field,this paper established the time-stepping finite element models of LSPMSM considering the influence of saturation,eddy current and other factors.These models took account of the permanent magnet field,the squirrel-cage asynchronous motor effect and the variable-frequency permanent magnet generator effect comprehensively and separately.It was found that the demagnetization field in the permanent magnet alternates even at a low speed,and reaches the largest demagnetization point close to the synchronous speed during the start with heavy load.When the LSPMSM starts without load,a larger demagnetization field appears only once at around half the synchronous speed.The further analysis revealed that the appearance of the demagnetization field at low speed and its alternate variation are due to the squirrel-cage asynchronous motor effect and the variable-frequency permanent magnet generator effect.The results of the calculation example verify the theoretical analysis.Finally,the experiment validates the creditability of the calculation results of the above finite element method.