The phase relationships of the Co-Mn-In ternary system at 673 K were investigated by X-ray diffraction, scanning electron microscopy and energy dispersion spectroscopy techniques in this study. The isothermal section consists of 9 single-phase regions, 16 two-phase regions and 8 three-phase regions. A new compound Co50-x-yMn25+xIn25+y was found at 673 K with a homogeneity range of x = 7-25 (32-50 at.% Mn) and y = -5-1 (20-26 at.% In), and its crystal structure was investigated by conventional and synchrotron X-ray powder diffraction techniques and transmission electron microscopy. The compound crystallized in a cubic NiTi2-type structure with space group Fd3m and the lattice parameter is a = 11.2405(2) angstrom for the typical composition of Co42Mn34In24. The measurements of M-T and M-H show that the novel compound has complex magnetic properties, which indicates coexistence of ferromagnetic and antiferromagnetic interactions. Magnetocaloric universal curves and Arrott curves show a characteristic of second-order transition. Critical behavior was investigated by modified Arrott plots technique, Kouvel-Fisher method and critical isothermal, which suggested that long-range interaction exists in this novel material.