High power water-cooled horizontal array diode lasers are critical components in laser cutting systems for the compact line beam shape. Because of heat spreading effect of heatsink and thermal crosstalk effect among the laser bars, local thermal resistance for each bar is different, leading to unhomogeneous output power for each bar and non-uniform intensity distribution for laser line beam. In this paper, heat transfer and pressure drop as function of water flow rate in a single bent microchannel is firstly studied by computational fluid dynamics. For reaching same local thermal resistance, numerical simulation is used to determine local heat transfer coefficient (HTC) distribution for microchannels within the horizontal array heatsink. The water inlet or outlet structure respectively in the upstream and downstream of distributed channels is designed to compensate the distributed pressure drop so that each flow path from inlet to outlet has same pressure drop. This work gave the basic thermal design rule for horizontal array diode lasers to reach uniform junction temperature. © 2018 SPIE.