There is a big difference in seepage law between the traditional well group and combined well group of single horizontal well and multi vertical wells for SAGD heavy oil recovery. The horizontal well can convert the injection-producer relation between the original vertical wells from a parallel plane one to a three-dimensional region one. Research on three-dimensional seepage mathematical model is based on potential theory. The seepage process of horizontal well was regarded as three-dimensional line sink and the ellipse potentiometric surface with the focus of the two ends of the perforation interval in the horizontal well was derived；the vertical wells were regarded as multiple planar point sources through longitudinal stacking and the cylindrical potentiometric surface with vertical well as the center line was derived；the mathematical model of the seepage pressure field and the pressure gradient field during the process of displacement in the composite well group was derived based on the coupling of the above results using potential superposition principle. Producing range of three-dimensional reservoir and the effect boundary of gravity discharge were determined. Effect of gravity on SAGD development effect，pressure field and pressure gradient field was analyzed. The results show that the gravity may enlarge pressure gradient field at far-wellbore zone or area with low pressure gradient produced by source and sink，which promotes the heated fluid in the oil reservoir to discharge to the horizontal wells.