The mechanistic model of cutting force for the dedicated cutter was established by using the method of the equivalent cutting edge vector, in order to accurately predict the changing law of the instantaneous cutting forces generated by the complex multi-tooth dedicated cutter during helical milling. Firstly, the kinematic characteristics of helical milling was analyzed, and the mathematic description of the motion path of any cutting position was achieved. Then the undeformed chips and machining surfaces generated at stable cutting stage were simulated. Thirdly, the length of the equivalent cutting edge, the depth of cut and the width of cut for its corresponding undeformed chip were calculated. Simultaneously, the peripheral cutting edge was analyzed and the whole simulated cutting force model was created. Finally, the simulation value of cutting force was solved through identification experiment of cutting force coefficient. The results show that the values above can accurately predict the instantaneous cutting force change law of the dedicated cutter, and the equivalent edge vector method is an effective way to realize the cutting force modeling of helical milling process.