Currently, a major hurdle preventing phosphorene from various electronic applications is its rapid oxidation under ambient conditions. Thus how to enhance its chemical stability by suppressing oxidation becomes an urgent task. Here, we reveal a highly effective procedure to suppress the oxidation of phosphorene by employing a suitable van der Waals (vdW) substrate and a vertical electric field. Our first-principles study shows that the phosphorene-MoSe2 vdW heterostructure is able to reverse the stability of physisorption and chemisorption of molecular O2 on phosphorene. With further application of a vertical electric field of -0.6 V Å-1, the energy barrier for oxidation is able to further increase to 0.91 eV, leading to a 105 times enhancement in its lifetime compared with that without using the procedure at room temperature. Our work presents a viable strategy to vastly enhance the chemical stability of phosphorene in air.