The invention discloses a primary sealing device for a floating roof tank and a mounting method of the primary sealing device. The primary sealing device is mounted between gaps at the edge of a floating plate; when the edge clearance of the floating plate is not changed, the sealing device is in a favorable fitting state with a wall of the tank under the effect of the elasticity of a sponge and a torsion force of a torsion spring; when the edge clearance of the floating plate is reduced, being supported by a rubber bearing pad, the sealing device is always in a perpendicular state, but the sponge is stressed to be in compressive deformation, so that the situation that the sealing device and the wall of the tank are in the favorable fitting state is guaranteed; and when the edge clearance of the floating plate is enlarged, the compressive force between the sealing device and the wall of the tank is reduced, and at this moment, a supporting rod can be bounced upwards under the effect of the torsion force of the torsion spring, so that a rubber wrapping belt and the wall of the tank are always in a close fitting state. When the supporting rod rotates, a lower sealing rubber belt can change in positions up and down along with the rising and falling of a fixing plate, however, no matter what condition the sealing device is in, the lower sealing rubber belt can always fit with the liquid level, so that the purpose of preventing petroleum gas from volatilizing is achieved.