The invention belongs to the technical field of online quantitative weighing and an automatic control system thereof, and in particular relates to a variable speed feeding quantitative weighing system based on a buoyant weighing and sensing principle. A mechanical system of the variable speed feeding quantitative weighing system consists of a feeding mechanism, a feeding machine, a weighing cylinder mechanism, a leverage which transfers a quantitative weighing force value during the weighing, a discharging mechanism, a discharging cylinder, a dowel bar, a buoyant weighing sensor and weights which can adjust the quantitative weighing value; and a programmable logical control (PLC) measurement and control system of the variable speed feeding quantitative weighing system consists of a chord length (L)/instantaneous velocity (V) variation and voltage amplification module, a filter module, an analog (A)/digital (D) conversion module, a PLC central processing module, a display module, a relay part, a feeding mechanism and an electromagnet control device. The basic principle of the system is that after the buoyant weighing sensor compares the measured weighing value with a preset weighing value range, when the output electrical signal value reaches the preset signal value, the PLC measurement and control system can be used for accelerating and slowing down the feeding machine, and thus, the dual purposes of increasing the production efficiency and improving the weighing accuracy are realized.