With the rapid development of aerospace field, ultrahigh temperature ceramics are required to have excellent mechanical properties in harsh environment, thus refractory metal carbonitrides have attracted much attention. In this work, the formation energy, lattice constant, volume, elastic constants, elastic modulus, hardness, fracture toughness, wear resistance, and density of states of refractory metal carbonitrides RMC1-xNx (RM = Zr, Nb, Hf, Ta) solid solutions have been systematically calculated by first-principles calculations. The formation energy results indicate that the stability of the RMC1-xNx increased with the increases of composition x, and the HfC1-xNx was the most stable at the same composition. The calculated lattice constant and volume of RMC1-xNx always decrease with the increase of N composition x, which also suggests that the stability of RMC1-xNx gradually increases. The RMC1-xNx solid solutions are mechanically stable. Furthermore, the addition of nitrogen could improve the brittleness and bulk modulus of the RMC1-xNx solid solutions. The impurity of nitrogen could enhance the fracture toughness of ZrC1-xNx and HfC1-xNx solid solutions. The calculated hardness and wear resistance of ZrC1-xNx and HfC1-xNx solid solutions firstly increase, and subsequently decrease with increasing N composition x. Finally, the results of the density of states also indicate that the stability of the RMC1-xNx increased with increasing N composition, which are also consistent with the energy results. In a word, our results are helpful for the design of metal carbonitride based ultrahigh temperature ceramics.