The invention provides an image size control method for an external drum type plate setter. The image size control method for the external drum type plate setter comprises the following steps that S1,the surface perimeter S of a printing plate, the rotating speed N of a roller and the frequency f1 of an encoder are obtained; S2, the pixel interval L is calculated according to the output resolution of the external drum type plate setter; S3, the scanning frequency f2 of a laser beam is calculated according to a formula (please see the specification for the formula); S4, the frequency doublingfactor T is calculated according to a formula (please see the specification for the formula): S5, the frequency doubling factor T is divided into the product of a frequency doubling parameter M and afrequency division parameter D according to a formula (please see the specification for the formula); S6, a module U1 with the frequency doubling parameter M and a module U2 with the frequency division parameter D are separately selected and sequentially connected with the encoder, so that a scanning frequency output module with a frequency f2 is obtained; and S7, with the output frequency f2 of the scanning frequency output module as the scanning frequency of the laser beam, scanning control is conducted in the plate setting process. According to the image size control method for the externaldrum type plate setter, the image sizes of printing plates can be adjusted according to different thicknesses of the printing plates, so that the problem that during CTP plate setting, image sizes output to the printing plates deviate from actually-required sizes due to different plate thicknesses is solved.