Traditional solid-solid phase change materials (PCMs) have fixed transition temperature due to their composition and structure determination, which impede their practical application in the field of thermal storage. In this work, according to the second law of thermodynamics and the phase equilibrium theory, PCMs can be further improved through form mixtures (molecular alloy) to regulate phase transition temperature and enthalpy for thermal energy storage. Herein, the binary molecular alloy of quaternary ammonium tetrachlorocuprate [1-CnH2n+1N(CH3)3]2CuCl4 (n = 16, C16C3Cu; n=18, C18C3Cu) as solid-solid PCMs were characterized over the entire composition range by DSC and XRD, and were further constructed phase diagram. Thus, a series of multiphase state materials of C16C3Cu-C18C3Cu, such as solid solutions (α, β, γ), intermediate compound ([1-C16H33N(CH3)3][1-C18H37N(CH3)3]CuCl4), and eutectoid compounds (α + γ, β + γ), could be selected from phase diagram to apply as PCMs with solid-solid phase transition temperature in the interval of 49 to 75 °C and enthalpy in the interval of 2.11 to 106.3 J•g−1, so as to extend the thermal performance of [1-CnH2n+1N(CH3)3]2CuCl4 as PCMs to adapt to thermal storage applications under different conditions.