Optical biosensors with the high sensitivity is an important tool for environment monitoring, disease diagnosis and drug development. Integrating the biosensor could reduce the size and cost and is desirable for home and outdoor use. However, the integrated structure always results in the worsening of sensitivity and narrowing of sensing range, especially for small molecule sensing. In this work, we propose an integrated plasmonic biosensor based on the resonant structure composed of dielectric grating and metal film. With vertically incident light from the grating side, the surface plasmon polariton (SPP) mode could be excited at certain wavelength and the reflected light would vanish. Simulation results indicate that, when varying refractive index (ndet) of detection layer, the energy of reflected light changes dramatically. Assuming the resolution of the power meter is 0.01dB, the sensing resolution could be 4.37×10-6 RIU, which is very close to the bulk lens based SPP biosensor by monitoring the light intensity variation. Since antibody and antigen always have the size of tens of nanometers, it is necessary to check the sensing ability of the sensor in tens of nanometers. Fixing ndet and varying the thickness of detection layer, calculation result demonstrates that the reflected light energy is sensitive to the thickness change with one hundred nanometers. This attributes to the surface mode property of SPP mode.