A viscosity reducer for oil-based mud and a method of reducing viscosity for the oil-based mud

A viscosity reducer for oil-based mud and a method of reducing viscosity for the oil-based mud are provided. The method includes steps of a) adding the viscosity reducer into the oil-based mud, b) injecting the oil-based mud into a well, c) recovering and sampling after the oil-based mud arrives the ground surface and testing performance of the oil-based mud, and repeating the step a) to the step c) until the viscosity of the oil-based mud reaches a required viscosity. The viscosity reducer is a fatty acid polycondensate, and the polycondensate is prepared by polycondensing one or more selected from a group consisting dodecahydroxyl stearic acid, polyhydroxystearic acid, stearic acid, a reaction product of the dodecahydroxyl stearic acid and stearic acid, and a stearic acid homopolymer. The invention also provides the method for reducing viscosity for the oil-based mud by use of the viscosity reducer for oil-based mud. The viscosity reducer for oil-based mud has good wetting and viscosity reduction performanc