The double differential cross sections for electron impact single ionization of helium at incident energies of 32.5, 36.5 and 40.7 eV are calculated by using the DS3C model. The calculated results are compared with the experimental date. The exchange effects are discussed. The single differential cross section for electron impact single ionization of helium is obtained by integrating the double differential cross sections over the angle of the ejected electron. The single differential cross sections of helium at incident energies of 32.5, 36.5, 40.7 and 50 eV are calculated by using the 3C model and the DS3C model. The calculated results are compared with the experimental date. The structure of the cross section is analysed and the contribution of exchange effect to the corss section is discussed.