A linkage device includes a shell, an active arm, a linkage lever, a random arm and at least two fixed poles. The shell includes two side plates and an upper plate interconnecting the two side plates. The active arm and random arm are both arranged horizontally under the upper plate and between the two side plates. The active arm and random arm each comprise a slide rail inclined to a horizontal surface. The slide rail of the random arm has an inclined direction opposite to that of the slide rail of the active arm. The linkage lever is arranged outside the shell. The linkage lever comprises a central portion pivoted to one of the side plates and two ends respectively pivoted to the active arm and random arm. Two of the at least two fixed poles respectively slide into the slide rails of the active arm and random arm.