The invention discloses a calculation method for a transmission line clearance spatial synthetic electric field under a mountain fire condition. According to the method, the height, temperature and form of a flame are obtained according to a typical vegetation burning test, a stable burning area is selected, and clearance below a transmission line is divided into a flame body equivalent area and a gas-solid two-phase body area; a difference moving particle sizing instrument is adopted to measure small particles, a vernier caliper is adopted to measure big particles, and the statistical law of particle volume concentration and particle size is obtained; the electric quantity qk of the particles in the flame is calculated; a simulation model is established, and a finite volume method is utilized to solve transient temperature distribution and fluid motion; simulation calculation is performed on particle movement in the flame clearance to obtain particle distribution in the flame clearance; particle positions and carrying capacity are set, and a finite element method is utilized to solve a spatial synthetic electric field to obtain electric fields E at different positions; and initial particle distribution and an initial electric field value are set, temperature-fluid-particle movement-electric field multi-field coupling calculation is performed, and a final spatial synthetic electric field is obtained. The method can provide a basis for research on the discharge mechanism and breakdown characteristics of clearance.