The invention relates to a high-precision performance detector of a resonant mode water level sensor and a test method thereof. In the high-precision performance detector, an air chamber pressure change is adopted to simulate an actual water level change; resonant mode water level sensor values in the air chamber are collected and then transmitted to a controller so as to be processed to obtain asensor performance curve; and the change of air quantity in the air chamber is equal to that of a liquid level in a water tank. The high-precision performance detector can be used for completely eliminating the defects of low stability, low precision and high error of the traditional test system. In the invention, firstly, a novel high-precision frequency measuring method is utilized so that the frequency measuring error of the system is not more than +/-5 Hz and the system precision reaches a new level compared with the errors of dozens of hertz even hundreds of hertz in the traditional testsystem; secondly, the system is fitted by sampled data through using a least square method and the system output curve is extremely optimized, so that a performance test curve of the sensor is more approximate to a real curve; and finally, a processor chip with a high integration level is adopted so that the reliability of the system is also obviously improved.