Terahertz wave detecting method based on multi-reflection optical lever with nano-scale displacement measuring precision is presented. Multi-reflection optical lever is composed of a pair of plane micro-mirrors, and detecting material is coated on one mirror to absorb terahertz radiation. Affected by radiant thermal effects, tiny mirror deformation is produced while displacement between mirrors is changed. These variations proportional to radiation are amplified and measured by multi-reflection optical lever, and then terahertz wave power can be obtained. Theoretical displacement measuring precision of multi-reflection optical lever method is better than 1 nm. Experimental results show that measuring stability of this method is better than Knife-edge filter method. This method achieves resolution of 4 nm, sensitivity of 5.95 nm/mV, and measurement range of 30 \mu m.