Obvious threshold pressure gradient exists during the development of low-permeability common heavy oil reservoir in Oudeh oilfield of Syria，which affects the producing degree of reserves. It is very important to study the influence of threshold pressure gradient on limit drainage radius of horizontal wells in the oil reservoir developed by natural energy and horizontal well. In order to comprehensively study the effect of the threshold pressure gradient on reservoir development law，first of all，the threshold pressure gradient was determined by the laboratory experiment，and its relation with mobility was known. Secondly，based on the quasi-three-dimensional ideas on ellipse flow in horizontal well，pressure gradient model of horizontal well in xy and yz plane considering threshold pressure gradient was set up. Finally，the limit drainage radius of the horizontal wells in oil reservoir was determined by using the method of steady state approximating non steady state.At the same time，the limit drainage radius chart of the horizontal wells in the quasi-three dimension was established. The research results show that the threshold pressure gradient of low-permeability common heavy oil reservoir is another major factor of low producing degree besides the factor of high viscosity of crude oil. The fluid flow of horizontal well in the xy plane is a linear flow，and that in the yz plane is a radial flow. The limit drainage radius of the horizontal well in the xy plane was larger than that in the yz plane. During the development by horizontal well in the low-permeability common heavy oil reservoir，the matching relationship between the reservoir thickness and the limit drainage radius of the horizontal well in the xy plane and yz plane should be considered fully in order to achieve the purpose of expanding the scope of producing reservoir.