Either nanowire or nanohole array for semiconductor were proved to be an efficient nanostructure to harvest solar light. However, for Si, the length of nanostructure about several micrometers is required to have acceptable absorption. Although this length already far less than the bulk Si in which hundred micrometers are required, the micrometers length still not feasible for Si nanostructure. High density nanostructures will cause extensive surface recombination that reduces the power conversion efficiency. Therefore, explore the dependence of light absorption to the length of Si nanostructure is very important to design an efficient solar cell. In this work, the Si nanohole array was fabricated in several depths from 110 to 960 nm. The total reflection was less than 1% at visible regime for 960 nm depth hole. The Ag nanoparticles were put at the bottom of the nanohole to explore the light absorption by plasmonic enhanced Raman scattering. A chemical, pNTP, was cover Ag nanoparticle as the prober for the plasmonic effect. As the laser light incident to the Ag nanoparticle, the surface plasmonic effect will enhance the Raman scattering of the pNTP. The enhanced Raman signal obtained from pNTP indicates the incident light could penetrate into the bottom of the Si nanohole array without significant absorption. The experiment result indicate the Raman signal decay fast after the depth of nanohole exceed 240 nm. This result indicate, the length of Si nanostructure may not need micrometers length to harvest incident solar light. This finding pave a bright route for design of Si solar cell with nanostructures.