A cable management arm supporting device is provided to be mounted between first and second slide rail assemblies. The first slide rail assembly includes first and second rails and a third rail movably connected therebetween. The cable management arm supporting device includes: two movably connected arms; first and second mounting members respectively connected with end portions of the arms and respectively mounted to the first and second rails; a supporting base including two supporting portions; two telescopically connected supporting members respectively connected with one supporting portion of the supporting base and the third rail; and a third mounting member movably connected with the other supporting portion of the supporting base and releasably connected with the second slide rail assembly.