The invention provides a crude oil viscosity reducer and a preparation method thereof. The crude oil viscosity reducer is a compound system composed of a humic acid (HA) and triethylene glycol condensate as shown in a formula I in the specification and a humic acid grafted polyether modified substance as shown in a formula II in the specification, wherein the mass ratio of the condensate as shown in the formula I and the humic acid grafted polyether modified substance as shown in the formula II is (20-40): (80-60). The crude oil viscosity reducer provided by the invention still has good viscosity reducing effect at a high temperature, and has good emulsifying and viscosity-reducing capacities for crude oil.