Rice straw, which is considered an excellent insulation material, can be filled into the hollows of concrete block after being pressed, thereby improving the thermal performance of the concrete block. This new type of straw-concrete composite block will have good mechanical and thermal properties. In this study, to explore the feasibility of this new type of block, the response surface method was introduced. The goal was to find the effects of processing parameters on the forming quality of straw blocks. The quadratic regression model was established, and the processing parameters were optimized. It was found that the forming density, vertical pressure, pressure-holding time, and the interaction between the forming density and pressure-holding time had significant effects on the forming quality of the straw blocks. The optimal conditions obtained by RSM optimization were a forming density, a vertical pressure, and a pressure-holding time of 319.7 kg/m2, 2.5 kN, 33.68 s, respectively. Under these conditions, the volumetric contractivity of straw blocks was 11.17%, the horizontal failure strength was 21.74 kPa, and the natural moisture content was 16.37%. The parameters calculated via the prediction model were highly consistent with the results produced via the actual measurements, which showed that the prediction model was reliable and potentially useful in guiding industrial production.