The present disclosure provides a hydraulic system of a cantilever device, which comprises an end cap, an adjustment brace, an adjustment bolt, a slider, and an upper pivot. The end cap comprises an upper wall, a bottom wall, and a side wall. The bottom wall has a securing groove. Each side wall has an upper pivot hole adjacent to the upper wall. The adjustment brace is fastened to the end cap and is secured in the securing groove. The adjustment brace comprises a track surface having a track slot. The adjustment bolt sequentially comprises a bolt head, a bolt neck, a bolt shoulder, and an adjustment portion. The slider comprises a first slider portion screwed to the adjustment portion, wherein the first slider portion has two shaft portions located on both sides of the adjustment portion. The upper pivot comprises a shaft body portion located between the bolt head and the bolt shoulder of the adjustment bolt. According to the cantilever device and the hydraulic system thereof, a situation where the current force adjustment device is unable to act stably can be improved through a configuration relationship among essential elements. The present disclosure further provides a cantilever device adopting the hydraulic system.