To reduce shading-loss by sidewalls of a flat-plate collector unit, the mathematic model of mean effective heat-absorption area coefficient was built by using geometric optics. The variations of annual mean effective area coefficient with aspect ratio (AR), spacing, tilt angle, and latitude were analyzed using this model. The results show that the mean effective area coefficient decreases as the spacing increase; especially for larger AR. It is adverse to improve the mean effective area coefficient when the tilt angle is less than 15° or greater than 75°. For convenient applied in practice, critical spacing values are given with over 0.90 of annual mean effective area coefficient. In view of the reasonable spacing of 4–6 cm for smaller convective heat-loss, the reasonable AR≤2/1 and the maximum plate spacing of 5.8, 5.4, 4.7 and 4.2 cm are recommended for a collector unit used in 20° N, 30° N, 40° N and 50° N regions. The corresponding annual effective area coefficients are greater than 0.90. In the design stages of the flat-plate collector unit, the convective heat-loss and shading-loss can be minimized based on the results of this work.