The effect of prestrain on tensile and compressive behaviors of single-layer black phosphorus (SLBP) in both armchair and zigzag directions is investigated by using molecular dynamics simulations. Prestrain is carried out by stretching or compressing SLBP in an orthogonal in-plane direction. The results show that the overall mechanical properties of SLBP, including Young's modulus, tensile strength, compressive strength, and yield strength are enhanced with an increase in compressive prestrain but are reduced with an increased tensile prestrain. With the same value of prestrain, SLBP exhibits unidirectional-homogeneous characteristics for tensile and compressive deformation. It is demonstrated that the armchair-oriented prestrain leads to a more significant improvement in the overall mechanical properties than the zigzag-oriented prestrain, indicating the anisotropic deformation behavior due to the characteristic puckers in SLBP. This work also reveals the mechanisms of prestrain underlying the mechanical behaviors of SLBP at the nanoscale, suggesting that the application of prestrain is an effective way to modify the mechanical properties of two-dimensional materials.