The sensing properties of pristine, B-, Al-, Si-, and S-doped blue phosphorus (BP) monolayer to nitric oxide (NO) are theoretically investigated using density functional theory and non-equilibrium Green's function method. We systematically discuss the concentration effect, sensing mechanism, and current-voltage (I-V) response of BP adsorbing NO molecule. Our results show that the pristine BP exhibits a weak sensitivity for NO molecule, while B-, Al-, and Si-doped BP strongly adsorb NO via robust chemical bonds, meaning a good potential in metal-free catalysts, but unsuitable as NO sensors. Interestingly, S-doped BP is recommended as a desirable material for NO detection due to moderate adsorption energy and large charge transfer. Besides, the flowing current in S-doped BP can be significantly improved after NO adsorption under the same bias voltage. Therefore, S-doped BP could be a good candidate as an NO sensor.