The invention discloses a rice field soil cadmium-arsenic synchronous passivator and a preparation method and application thereof. The passivator contains three layers structures from inside to outside, and each layer is provided with an independent coating film; the innermost layer is a hydrogen ion chemical consumption agent, the middle layer is a reaction accelerator humus substance, and the outermost layer is a mineralization precursor; the hydrogen ion chemical consumption agent is nitrate and peroxide; the reaction accelerator humus substance is more than one or more than two of peat soil, humic acid, fulvic acid, humin, yellow humic acid, nitrohumic humic acid and black humic acid; and the mineralization precursor is reducing iron powder, ferrous salt and/or solid ferrous ore. According to the cadmium-arsenic synchronous passivator with the three-layer film structure, effective components can be slowly released into the soil, the effect can be fully played, so that violent reaction or agglomeration of the components is avoided, the passivation efficiency is improved, the content of cadmium and arsenic in the heavy metal of the rice can be effectively reduced, and the standard production of the rice field in the medium and light pollution can be realized.