The polarization of light, the vector nature of electromagnetic waves, is one of the fundamental parameters. Finding a direct and efficient method to measure the state of polarized light is extremely urgent for nano-optical applications. Based on Malus's law, we design an ultracompact metasurface composed of silver nanorods, which is demonstrated to directly measure the state of linear polarization by a grayscale image. Using an ultrathin metasurface, we generate grayscale images with gradient grayscale levels which are linked directly to the polarization state of the incident light. The direction of the linear polarization of incident light can be conveniently and efficiently obtained through extracting the angle of the brightest area of the grayscale image. The ultrathin metasurface operates in the broadband 750-1100 nm spectral range. It is a novel and significant method to analyze the linear polarization state of light, which provides opportunities for various applications, such as polarimetric multispectral imaging and miniaturized polarimeter.