Based on the requirement of the laser guidance weapon platform with small caliber and large instantaneous field of view, the scheme of a small strapdown defocused refraction optical detection system was proposed. According to the target detection model of strapdown semi-active laser guidance and the ideal defocused optical system model, the optimal system parameters were obtained with the consideration of the application of laser guidance. Based on these parameters, ZEMAX was used to optimize the defocused optical detection system with the work wavelength of 1 064 nm. The critical effective detection half field of view of this system were 5.89 degrees, the critical detection half field of view were 13 degrees, and the length diameter ratio was 1.27. The aberration from the dome was solved well. The linear relationship between spot centroid position and target position deviation angle was presented. The experimental results show that the error of the designed optical defocused detection system is less than 0.15 degrees, meeting the requirement for target position deviation angle detection of large field of view.