The definition of tort liability in third-party environmental pollution control cannot rely on the form division of entrusted management code, custody operation mode, centralized management mode and decentralized governance mode. It should be carried out from the clear relationship between the polluting and pollution control enterprises. According to the pollution control facilities provided by the polluting or pollution control enterprises, the basic relationship between the polluting and pollution control enterprises in third-party environmental pollution control may be the legal relationship of environmental service contracts or work contracts; in addition, the basic relationship between the two may also be an invalid contractual relationship. Under the legal relationship of environmental service contract, the pollution control enterprises should assume joint and several liability with the polluting enterprises if there is any fault in the former. Under work contract, the pollution control enterprises shall bear tort liability if there is no ordering, directing or choosing negligence in the polluting enterprises. Under invalid contract, common infringement, common dangerous behavior, separate infringement with cumulative causality and separate infringement with joint causality should be distinguished, for the polluting and pollution control enterprises to assume joint, or shared, or partly joint and partly shared tort liability. The ultimate liability between the polluting and pollution control enterprises shall be allocated according to the appointed or legal proportion.