Abstract: This paper describes the function and states of the evaporator unit of boron recycling system of Ling-ao Nuclear Power Plant. Based on the state commissioning of the evaporator unit of boron recycling system, the items for states commission, the formula used in the commissioning of evaporator unit, and the state parameter curves of the evaporator from normal starting-up to sequential stopping states are presented. The origin and corrective measures for the problems occurred during the commission are also presented in the paper. The commission shows that the operation of the evaporator unit of boron recycling system can fully meet the design requirement.