Boron monolayers have been increasingly attractive, while it is still a challenge to understand their structural stabilities, due to electron deficiency and multi-center bonds. In this work, we propose the average electron compensation (AEC) mechanism for boron monolayers based on high-throughput first-principles calculations. It is found that the AEC parameter (λ) tends to be zero for the stable free-standing boron monolayers. In addition, this mechanism can quantitatively describe the stability of boron monolayers on various metal substrates, providing direct suggestions for experimentalists to synthesize various boron monolayers for practical applications.