Abstract: The poor suitability of windback seals existed in their application for the D-type oil pumps under different operating conditions when different kinds of crude oils were delivered, which resulted in bad sealing effect. By analyzing the suitability problem, the author considers that the parameters like viscosity of the crude oil transported, operating temperature and pressure should not be taken as constants in the design of windback seal, otherwise, the leakage from the windback seals will not be avoided. Therefore, the design parameters to be selected for windback seals should be determined according to the actual operating conditions of pumps. In this paper, a successful design of windback seal has been given by the author, the leakage amount has been reduced to certain degree.