Motors comprise the major electricity loads in offshore oil projects and motor starting may lead to instantaneous impacts. The impacts probably deteriorates system performance if DC distribution networks are employed. Based on the idea of analytical expression,the impacts of such loads are analyzed. Firstly,by simplifying the differential algebraic equations of the system,the analytical expression of DC bus voltage is derived. Then the impacts of instantaneous active load on DC distribution network in offshore oil platform are researched. Finally,a four terminal DC distribution network model based on the PSCAD/EMTDC platform is built and different instantaneous loads are set for simulation. The DC bus voltage under different instantaneous loads is analyzed. The correctness of the analytical expression is verified. The results show that the analytical expression effectively reflects the adaptability of instantaneous load on DC distribution network in offshore oil platform.