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A model for the evolution of two droplets laden with insoluble surfactant coalescing on a preset film is established according to the lubrication theory, and the coalescence processes are simulated. The role of the surfactant and its inherent mechanism are investigated, the effects of the Marangoni number, the preset liquid film thickness and the initial spacing between the two droplets on the coalescence are examined. The results show that the droplets encounter each other, gradually overlap, and finally coalesce into a "new" droplet. The Marangoni effect is beneficial to the convergence of the two inner leading fronts of the droplets in the early stage, but it hinders the accumulation of the droplets and subsequent coalescence. Increasing the Marangoni number promotes not only the aforementioned inhibition, but also the convergence of the two leading fronts towards the center, which speeds up the coalescence of the surfactant. Moreover, the diffusion of the surfactant towards the outsides of the droplets is accelerated; hence, its distribution along the droplet surface is more uniform after the coalescence. The droplets and the surfactant undertake a longer "journey" to achieve coalescence when their initial spacing is increased; increasing the preset film thickness shortens the time of coalescence required. [ABSTRACT FROM AUTHOR]