Metasurfaces consist of arrays of artificial atoms arranged on a subwavelength scale, and have significant advantages in modulating the phase, amplitude, and polarization of optical field. Limited by the discrete sampling principle and the assumption of periodicity, the conventional forward design method suffers unavoidable design errors, which easily leads the device performance to degrade. In this paper, a freeform wavelength division multiplexing (WDM) metagrating with a large deflection angle and polarization-insensitive characteristics is inversely designed by using an adjoint multi-objective topology optimization method. The simulation results show that the topology-optimized WDM metagrating has superior polarization in sensitivity compared with the discrete regular structure, with a deflection angle of 70.8° at 510 nm, an absolute deflection efficiency of 48%, and a transmission efficiency of 98% for 852 nm incident light. On this basis, the absolute deflection efficiency can be optimized to more than 70% by using a random initial structure. The freeform WDM metagrating designed in this paper has the advantages of large deflection angle, high efficiency, and low spatial crosstalk, and has potential applications in optical communication, micro and nano-optical field modulation, and Rydberg atom-based microwave measurements.