A nano-indentation ultrasonic detecting system for detecting mechanical properties of a target material. An indentation device, disposed on a surface of the target material, generates an indentation on the surface, obtaining the relation between the Young's modulus and the Poisson's ratio of the target material. An ultrasonic generator is movably disposed on the surface of the target material, generating at least two different ultrasonic signals thereon. An ultrasonic receiver is disposed on the surface of the target material and separated from the ultrasonic generator, receiving the ultrasonic signals. The result of a nano-indentation experiment are applied in the ultrasonic theory and iterated by the ultrasonic experimental data and theory, obtaining the Young's modulus of the target material. The obtained Young's modulus of the target material is substituted back in the result of the nano-indentation experiment, obtaining the Poisson's ratio of the target material.