This paper describes a shear horizontal surface acoustic wave（SH-SAW）immunoassay biosensor on a quartz substrate. The SH-SAW biosensor is a 250MHz SH-SAW delay-line which has a gold surface covered with antibodies. It is important to discuss the detection mechanism of SH-SAW biosensors on quartz. In this paper, the detection mechanism of SH-SAW biosensors are investigated, using three numerical analyses. The first analysis is based on a mass loading model. The second analysis is based on a viscosity model. The third analysis is based on a viscoelasticity model. The numerical calculation method for quartz-based SH-SAW propagation characteristics, which is a modified Campbell and Jones method involving the effect of liquid viscosity is described. In order to confirm the proposed numerical analysis models, 250MHz SH-SAW biosensors on quartz substrates were tested. The mass, viscosity, and viscoelastic behavior of fluids on SH-SAW biosensors were theoretically and experimentally discussed.