The primary emulsifier for oil-base mud MOEMUL-S is a low molecular polymer with a variety of surface active groups synthesized by molecular structure design.lt can be used in conjunction with MOCOAT-S CO emulsifier in the field to form an adsorption film with a certain strength at the oil-water interface, reduce the tension at the oil-water interface and increase the viscosity of the external phase. In this way, the dispersion phase droplets are prevented from coalescence and becoming larger, and the stability of oil-base drilling fluid is significantly improved. The drilling fluid still has good performance at high temperature and high density. MOEMUL-S and its auxiliary emulsifier MOcOAT-S have stable rheological properties and high temperature and pressure filtration performance before and after rolling at high temperature, and strong resistance to various pollutants. lt is an excellent main emulsifier for oil-base drilling fluid system.