The aberration characteristics change with the relative position changes of zoom groups has been a key problem for continuous zoom lens design, especially those with large target surfaces. To diminish the aberration, in this study, we took a Positive-Negative-Positive-Positive（PNPP）Mechanical structure for optical compensation, and developed a continuous zoom MWIR lens system with 20x magnification. The system used a cooled detector with a large target surface of 1280×1024 pixel and a 12μm pixel size, which improved observing capability under a 66.5° field of view. We tested the transfer function of the system at Nyquist frequency, which is close to the diffraction limit in a 15~300mm zoom range and the full field of view. We further evaluated our system by ray tracing analysis, pressure angle analysis of the cam curve, and de-focusing amount analysis under a temperature range of -45℃ to +70℃. Results show that our system has a good suppression capability for cold reflection, high image quality under extreme thermal conditions, and a smooth zoom curve. © 2023 SPIE.