Adhesion and bonding are critical to the success of polymer composites, particularly in the sealing processes of redox flow batteries, where laser transmission welding technology for plastics is beginning to gain traction. However, the key factors influencing welding and microscale bonding mechanisms remain incompletely understood. In this article, we systematically analyzed the optimal process parameters and patterns for laser welding of polypropylene (PP) plastics using orthogonal experiments and investigated the interfacial bonding mechanisms through morphology analysis and fracture force testing. The results indicate that an optimal adhesion and sealing performance can be achieved when the laser transmittance is approximately 40% and the line energy density is in the range of 7–8 J/mm. We categorized three distinct regions: heat-affected zone (HAZ), melt flow mixing zone, and core zone, which significantly enhance welding bond strength. Among these, the core zone, characterized by a dense fibrous bonding layer with a flocculent structure, contributes the most to bonding strength, forming a microriveting enhancement mechanism. The melt flow mixing zone features irregular large cavities that provide a mixing interlock effect, while the sparse flake-like bonding layer in the HAZ represents pseudo-adhesion characteristics present on the interface surface. Furthermore, raised-platform welding with enhanced mixing features was also explored, offering distinct insights into improving the welding bond strength. This study elucidates the essential factors and mechanisms of laser welding adhesion, and the optimized process parameters will significantly enhance the bonding level of the PP plastic interfaces. Additionally, this research provides additional perspectives on prevention and control strategies for welding seal failure caused by friction shear in the energy industry.