The invention discloses a mechanical transmission mechanism of a double-material tank fertilizer applicator. The mechanical transmission mechanism comprises a double-material tank, wherein a motor is arranged at the bottom of the double-material tank; the motor is fixed on a support plate on a side surface of the double-material tank; outer chain wheels 1, chain wheels 2 and stirring chain wheels are arranged on support plates; the chain wheels arranged on the support plates on two side surfaces are of a symmetric structure; an organic fertilizer rotating shaft is arranged between two chain wheels 1; a composite fertilizer rotating shaft is arranged between two chain wheels 2; a stirring shaft is arranged between the two stirring chain wheels. The mechanical transmission mechanism is of a structure that three groups of chain wheels are driven simultaneously by one motor and three shafts are driven to rotate simultaneously. The mechanical transmission mechanism is compact and simple in structure, the working and operation cost and the manufacturing cost are lowered, the fertilizer application efficiency is effectively improved, and moreover the structure is convenient to mount and repair and is applicable to operation under harsh environment conditions.