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 methods of numerical analysis. The first method is a mass loading model analysis. The second method is a viscosity model analysis. The third method is a viscoelasticity model analyasis. A numerical calculation method for SH-SAW propagation characteristics, which is a modified Campbell and Jones method involving the effect of liquid viscosity is applied to quartz. 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.