Developing a non-toxic tellurium-free alternative for PbTe is crucial for large-scale waste heat recovery. Here we demonstrate that a simple Cu3SbSe4-CuAlS2 composite can reach a peak zT of 1.8 and an average zT of 0.77 in the temperature range of 300-723 K, both of which are the record-high values in diamond-like semiconductors. Experimental transport data and X-ray absorption fine structure spectroscopy analysis suggest that the significantly enhanced electrical performance mainly originates from the diminished carrier scattering deformation potential. More importantly, Cu3SbSe4-CuAlS2 composites exhibit a weak temperature dependence of compatibility, making it superior to most IV-VI thermoelectric compounds from the aspect of device fabrication. Meanwhile, both the thermal stability and mechanical performance of Cu3SbSe4 are improved by CuAlS2 compositing. Our findings reveal that the Cu3SbSe4-based composite made of environmental-friendly and earth-abundant elements can be comparable to p-type PbTe in middle-temperature thermoelectric power generation.