Traditional PV/T systems, with passive cooling channels, can not solve the problem of coupling power/heat source on the surface of PV modules, resulting in lower electrical efficiency of solar cells. The active spectrum regulation technology using nanofluids, is a promising method to absorb spectrum energy not responding to solar cells, and reduce cell temperature and improve electricity efficiency. Though many nanofluids have been selected as optical nanofluids to separate/decoupling electricity and heat from composite spectral energy, no feasible method was proposed to select proper nanofluids to match the ideal window of solar cells. Therefore, from the view of spectrum regulation, some blended Ag nanofluids were present to numerically investigate the performance of PV/T systems, using a 2D-Monte Carlo method. Results indicated that nanoparticle radius, ranging from 20 nm to 60 nm, drove the movement of peak absorption from 395 nm to 520 nm, following a linear profile. Meanwhile, increased volume concentration and optical thickness reduced spectral transmittance, leading to lower cell temperature but worse output performance. Additionally, blended Ag nanofluids, with particle radius of 20 nm or 20/40 nm (8:2), volume concentration of 2.5 ppm and optical path of 10 mm, were optimal solutions for both Si cell and GaAs cell. The electrical efficiency and merit function value of Si cells were 11.85% and 1.61 for 20 nm nanofluid, 11.0% and 1.66 for 20/40 nm (8:2) nanofluid, while that of GaAs cell were 9.30% and 1.92 for 20 nm nanofluid, 9.03% and 2.05 for 20/40 nm (8:2) nanofluid, respectively.