The effects of small organic molecule (SOM) adsorption with benzene (C₆H₆), hexafluorobenzene (C₆F₆), and p-difluorobenzene (C₆H₄F₂) on the electronic properties of stanene under external electric fields are investigated through first-principles calculations. Different adsorption sites and molecular orientations are considered to determine the most stable configurations of small organic molecule (SOM) adsorption on the surface of stanene. The results show that the internal electric field caused by the adsorption of small organic molecules destroys the symmetry of the two sublattices of stanene in C₆H₆/stanene, C₆F₆/stanene and C₆H₄F₂/stanene systems with the most stable configurations, opening the band gaps of stanene with 39.5, 18.9 and 14.5 meV, respectively. Under an external electric field, a wide range of linearly tunable and sizable direct band gaps (31.6–420.1 meV for the C₆H₆/stanene system, 14.8–587.2 meV for the C₆F₆/stanene system and 14.5–490.2 meV for the C₆H₄F₂/stanene system) are merely determined by the strength of the composite electric field despite its direction. The mechanism of charge transfer between stanene and organic molecules under an external electric field can be revealed using an equivalent capacitor model to explain the tunable charge transfer. More importantly, the high carrier mobility of the stable SOM/stanene systems under an external electric field is largely retained due to the weak interactions at the interface. These results indicate that the electronic properties of stanene can be effectively modulated by the surface adsorption of organic molecules under an external electric field, providing effective and reversible routes to enhance the performance of stanene for novel electronic devices in the future.