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This paper presented a kind of body fluid density sensor based on the shear horizontal surface acoustic wave (SH- SAW) generated and detected by interdigital transducers (IDT). When the body fluid comes in contact with the delay path in a SH-SAW device, the viscous coupling action will effect on the wave propagation. Consequently the frequency, phase and propagating velocity of the SH-SAW will be altered. The theory analysis and initial experiment are presented. The result shows when signal frequency is 57.8 MHz, the SH-SAW can be excited effectively in the lithium niobate crystal. By measuring the phase change of the SH-SAW, the body fluid density can be detected. Compared with traditional body fluid density sensor, the SH- SAW body fluid density sensor has the advantages of simple construction, high precision and little sample etc.