In the present work we have studied the double differential cross sections for single ionization of a helium atom by electron impact at low energies. The double differential cross sections of the secondary ejected electron have been obtained by integrating the triple differential cross sections over the entire angular range of the primary scattered electron. The double differential cross sections for electron impact ionization of helium at incident energies of 26.3，28.3，30.3，32.5，34.3，36.5 and 40.7eV have been calculated in the BBK and DS3C model. The present DS3C results are compared with the absolute measurements，and reasonably good agreement has generally been found except for low incident energies (e.g.，26.3eV) and low ejected angles. In addition，the direct and exchange amplitudes are also considered. It has also been shown how the exchange effects contribute to the cross sections.