Giant tunneling magnetoresistance (TMR) has always been a pursuit in the research of magnetic tunnel junctions (MTJ). Two-dimensional (2D) magnetic materials have been used to construct lateral MTJ with high TMR. Here we investigated the crystal structure and magnetic property of CrSI monolayer, and found that it is a ferromagnetic semiconductor with a Curie temperature of about 180 K. The CrSI monolayer with Li adsorption (Li-CrSI) show ferromagnetic half-metallic with a high Curie temperature of 300 K. Further we designed a lateral Li-CrSI/CrSI/Li-CrSI monolayer MTJ. The TMR of the MTJs along b transport direction is 3 orders of magnitude times larger than that of the a transport direction, which should result from the different spin filtering ability along the two directions. The TMR of b transport direction in the MTJ is 7.67 × 10¹⁴, which is significantly higher than that of any reported lateral MTJs based on 2D materials. Our results provide a promising avenue for designing lateral MTJs with giant TMR and high Curie temperature.