The invention discloses a measurement system of a relative altitude and a relative attitude of an air vehicle and a measurement method thereof. Four ultrasonic ranging modules are installed at different positions of the air vehicle, ultrasonic wave emitting and receiving of the ultrasonic ranging modules are controlled through an MPU, ultrasonic wave transmission time is calculated, pressure parameters, temperature parameters and humidity parameters of atmosphere are collected so that the transmission speed of the ultrasonic waves can be compensated, meanwhile by means of a range error compensation module, range errors of an ultrasonic ranging sensor are compensated, relative height data of the air vehicle and the ground are measured, a relative height calculation model and a relative attitude calculation model are built, and the precise relative height between the centers of wings of the air vehicle and a landing runway plane and attitude angle information of an ultrasonic installation plane relative to the runway plane are calculated. The measurement system and the measurement method improve precision and stability of measurement of the relative height and the relative attitude during landing of the air vehicle.