Molecular dynamics simulation with the Stillinger-Weber potential is employed to study the mechanical behaviors of single-layer black phosphorus (SLBP) at different temperatures. The SLBP is initially loaded by a uniaxial tensile/compressive prestrain in either armchair or zigzag direction then stretched or compressed in the direction that is orthogonal to the direction of the prestrain. It is found that mechanical properties such as tensile/compressive Young's modulus, tensile/compressive strength and yield strength are enhanced by a compressive prestrain but are reduced by a tensile prestrain, and that an armchair-oriented prestrain can improve mechanical properties more significantly than a zigzag-oriented prestrain. The enhancement in mechanical properties by a compressive prestrain becomes less significant as the temperature increases. In addition, both yield and compressive strengths of SLBP are found to be highly sensitive to its size while this effect on the modulus and tensile strengths is quite limited. The mechanisms of prestrain underlying the mechanical behaviors of SLBP are also clarified in detail. Numerical results indicate that the application of a compressive prestrain is an effective way to achieve improved modulus and strength for two-dimensional materials.