The invention discloses an oil gas collecting device of a crude-oil outward floating roof tank. The oil gas collecting device comprises sealing cord fabric. The sealing cord fabric is connected between a tank roof and a floating disc, a tank-roof pressing strip plate is arranged on the tank roof, a floating-disc pressing strip plate is arranged on the floating disc, the upper edge of the sealing cord fabric is connected with the tank-roof pressing strip plate, and the lower edge of the sealing cord fabric is connected with the floating-disc pressing strip plate; and sealed oil gas collecting space is formed among the sealing cord fabric, the tank wall and the floating disc and connected with an oil gas recycling system through an oil gas recycling pipe, and breather valves are arranged on the tank-roof pressing strip plate. According to the oil gas collecting device, as the sealing cord fabric is arranged between the tank roof and the floating disc, the contact between oil products and air can be completely isolated, all volatilized gas is entirely delivered to the oil gas recycling system in a sealed manner, overflowed oil gas can be prevented from being directly discharged to atmosphere, and the environment is protected; and as no oil gas overflows, the danger level of the surface space of the floating disc is reduced, and the safety level of the oil tank is improved.