Abstract: Aiming at the problem of low heat transfer efficiency of traditional roll-bond flat plate, this paperconducts a numerical simulation study on the flow and heat transfer characteristics in the "Y"-shaped channel unit.The four parameters of width, length, runner parting angle and runner width are used as control variables to optimizethe heat transfer structure of the runner. Taking the "Y"-shaped channel unit as the research object, the finite ele-ment model of conjugate heat transfer was established by using Fluent. Through numerical simulation analysis, it isconcluded that in the defined length and width interval, the thermal efficiency increases first and then decreaseswith the increase of the length of the heat transfer unit, and the pressure drop coefficient has a negative correlationwith the length and width of the unit. The runner width has a positive correlation with the thermal efficiency and anegative correlation with the pressure drop coefficient. Then, according to the Box-Behnken principle, a responsesurface test for heat transfer optimization was designed, an optimization model with thermal efficiency and pressuredrop coefficient as the response indicators was established, and the optimal element design scheme was determined.The optimization results show that when the subunit length of the heat transfer runner is 40 mm, the width is 35 mm,the parting angle is 60°, and the runner width is 11 mm, the thermal efficiency of the "Y"-shaped runner is 0.782,and the pressure drop coefficient is 0.121. Finally, a direct-expansion solar auxiliary heating system is built to testthe scheme. The experimental results show that the collector using the "Y"-shaped channel shows better solar ener-gy utilization, which increases the COP of the system by 15.2%. This study provides a reference for the design andoptimization of other types of flow channels to improve the heat transfer efficiency of the roll-bond flat plate.