This paper analyzed the reason of drain in block in No.3 low pressure (LP) heater in 220 MW units during the startup and debugging stage. Based on the real-time data, the original design parameters and the site situation, the drainage systems in No.2, No.3 LP heaters were modified, and the cooling water of slag cooler was adjusted, in order to solve the drain in block in No.3 LP heater. Finally, this paper suggested the solution for the influence of cooling waters backwater temperature in slag cooler on the No.3 LP heaters drain