The traditional causality theory of “all or nothing” is inclined to cause such consequences that victims fail to claim or infringers over?compensate in cases of indemnity for medical damage while being used to handle cases concerned， which is against the purpose of the Tort Liability Law—to remedy damage. This study was originated from the doubt on the reasoning of a medical damage compensation judgment. By combing and demonstrating the feasibility and typification of proportional liability applicable to medical damage infringement， it put forward the determination rule of proportional liability as the medical damage liability in the case of unclear causal relationship.