Based on the thermojunction assignment test, actual thermal cycling curve of welded seam heat-affected zone is obtained. After simplifying high-temperature plasma as dot heat reservoir, heat reservoir of light gauge welding as linear heat reservoir and adopting principle of superposition, mathematical model to laser welding zinc coated high strength steel is established and analyzed, meanwhile theoretical thermal cycling curve of welded seam heat-affected zone is obtained. By contrasting the two results, it demonstrates that thermal cycling mathematical model can explain actual thermal cycling process of deep penetration laser welding. Microscale martensite is existed in welding seam microscopic structure, which indicates that thermal cycling characteristic of laser welding is quick heating and cooling velocity.