In this paper, a low crosstalk 3-LP mode 12-core fiber based on heterogeneous structure is designed. The core adopts a heterogeneous ring refractive index distribution without trench-assisted structure, which is simple in structure and can increase the effective mode area of the core. COMSOL software is used to analyze the performance of crosstalk and effective mode area of heterogeneous cores. The results indicate that the inter-core crosstalk of the heterogeneous core LP01, LP11, and LP12 modes are lower than -0.78, -0.66, and -0.4 dB/km, respectively, and their effective mode areas are 150, 166, and 200 μm 2, respectively. With a square lattice core arrangement, a low crosstalk 3-LP mode 12-core fiber design with a cladding diameter of about 213.8 μm and a relative core multiplexing factor of 26.9 can be realized, providing device support for the upgrade and expansion of communication capacity.