High entropy ceramics are the research hotspot in recent years. Transition metal boride medium entropy and high entropy ceramics have become important candidate materials for extreme environment resistance, due to their excellent mechanical properties, chemical reaction inertia and high melting point. In this work, the effects of B₄C excess content on the densification, microstructure and high temperature flexural strength of medium entropy boride ceramics were studied for the first time, and the preparation process of (Ti,Zr,Hf)B₂ powder with low oxygen content and high sintering activity was synthesized. The entropy ceramic density of (Ti,Zr,Hf)B₂ prepared by hot pressed sintering method at 1800℃ is over 99%, and the grain size of (Ti,Zr,Hf)B₂ ceramics with excess content of 15 wt% B₄C is 5.0±2.1μm. As the excess content of B₄C increases to 25 wt%, the grain size is significantly refined to 2.4±0.7μm. The part of excess B₄C reacts with Si₃N₄ introduced by ball milling, to produce BN phase by in situ reaction, while the other part of B₄C exists in the matrix as a second phase. The introduction of BN and B₄C phases can effectively suppress grain growth during the sintering process of medium entropy ceramics, while also improving the high-temperature flexural strength of the ceramics. When B₄C exceeds 25 wt%, the (Ti,Zr,Hf)B₂ medium entropy ceramics have the highest high-temperature flexural strength at 1600℃ of 631±62 MPa.