Biomass is generally regarded as a significant renewable resource, which can be used as the raw material for bioproducts. In the study, a mixture of rice straw and fermented cow manure was utilized to produce potted seedling trays, which are biodegradable and environmentally friendly. The effects of process parameters on the quality of final products were assessed by measuring damage resistance (DR) and expansion ratio (ER). Single-factor experiments and the response surface methodology (RSM) were used to optimize process parameters, including moisture content, forming pressure, mass percentage of straw (the ratio of rice straw to the mixture), and forming temperature as variables, and DR and ER of potted seedling trays as responses. The results showed that moisture content had the largest effect on DR and ER, followed by forming pressure, forming temperature, and mass percentage of straw. The optimal conditions were identified as a moisture content of 13%, forming pressure of 124 kN, mass percentage of straw equal to 8%, and forming temperature of 132 °C. There was a good agreement between the experimental data and the predicted results, indicating the reliability of the optimization process. Under the optimal conditions, the effectiveness of the regression model was further validated by the desirability of 0.97. Our findings shed new light on the perfect utilization of straws and animal manure and provided a reliable reference for the preparation of potted seedling trays from other types of biomass produced by agriculture.