High-strength galvanized steel is increasingly applied in automobiles according to its excellent mechanical property and corrosion resistance. Hybrid joining of high-strength galvanized steel and polymer is advantageous to the lightweight design of vehicles. Injection molded direct joining (IMDJ) is a promising hybrid joining method. IMDJ first forms small structures on the metal surface and then injects melted polymer onto the metal surface via injection molding. In our previous study, we applied hot water treatment to high-strength galvanized steel for the first time and successfully obtained the joining of treated steel and polybutylene terephthalate via IMDJ. However, hot water treatment condition needs to be controlled accurately to ensure the joining performance and the Zn coating undamaged. In this study, in order to optimize the hot water treatment condition and obtain better joining performance, several characterization methods were carried out. Surface chemical elements with different acid washing time were detected by X-ray photoelectron spectroscopy (XPS). The result shows the passive film can be removed clearly and Zn coating can fully contact with water after galvanized steel dipped in 10% HCl for 10 seconds. Scanning electron microscope (SEM) was conducted to observe the surface texture of treated galvanized steel under different conditions. The results show that needle-like nanostructures started to generate after 5 minutes by hot water treatment. The cross section of treated galvanized steel was also observed by SEM. The Zn coating was confirmed to be unbroken and completely covered the steel even after hot water treatment. Tensile shear test was performed to study the effects of treatment condition on joining strength. The experimental results show that acid washing process has a significant influence on joining strength. When the treatment time was 40 minutes, the optimized joining strength can reach above 20 MPa.