A piezoelectric biosensors for detection of plasm viscosity is developed, which is based on liquid damping effect of quartz crystal resonator. The measuring principal is given. Simultaneously, the detector of sensor and oscillator circuit is developed. The test results show that the maximal frequency shift of sensor are linearly dependent on square root of viscosity. The proposed sensor is much more simpler, more sensitive than the traditional methods. Furthermore, The sensor can be used to on-line detection.