In view of the limitation of traditional plasma optical sensors, and being capable of enhancement of absorbing infrared waves by magnetic resonance, a multiresonant nonplasma optical sensor based on black phosphorus (BP) is proposed. A BP monolayer and a simple dielectric structure are adopted by the sensor, Due to the anisotropy of BP, the sensor can achieve different sensitivities and figure of merits (FOMs) in different crystal directions. Through optimizing the sensor structure, two resonant peaks which are very narrow are obtained in different directions of BP crystals around 8 μm wavelength, and the sensitivity of the sensor is improved by the relatively low full width at half maximum. The highest absorption rate reaches up to 99.6%, while the highest sensitivity reaches up to 180 nm/RIU and FOM reaches up to 261 nm/RIU.The simulation results are verified by the coupled mode theory, and the physical mechanism is revealed. The sensor structure has great potentials for the future optical sensing application.