A hydraulic system switchable between fixed-displacement and fixed/variable-displacement, comprising: an oil tank, comprising an oil supply port and an oil return port; a working pump, comprising a displacement changing mechanism for changing a displacement of the working pump, wherein an oil inlet of the working pump is in fluid communication with the oil supply port of the oil tank; a control cylinder, used for controlling the displacement changing mechanism; a working oil line, in fluid communication with an output main path of the working pump; a load-sensing system, comprising a load-sensing circuit which outputs hydraulic pressure based on a load; and a load-sensing valve and a fixed/variable-displacement switching control valve, wherein a first control port and a second control port of the load-sensing valve are respectively in fluid communication with the output main path and the fixed/variable-displacement switching control valve; both the working oil line and the load-sensing circuit are in fluid communication with the fixed/variable-displacement switching control valve; the load-sensing valve is in fluid communication with the output main path, the oil return port of the oil tank and an input port of the control cylinder; the fixed/variable-displacement switching control valve is switchable between a first valve position and a second valve position, when the fixed/variable-displacement switching control valve is in the first valve position, the load-sensing circuit is in fluid communication with the second control port of the load-sensing valve, when the fixed/variable-displacement switching control valve is in the second valve position, the working oil line is in fluid communication with the second control port of the load-sensing valve, to control the working pump to switch between a fixed displacement state and a fixed/variable displacement state; when a differential pressure between input pressure of the first control port and input pressure of the second control port is greater than a first threshold, the output main path is in fluid communication with the input port of the control cylinder; when the differential pressure between the input pressure of the first control port and the input pressure of the second control port is less than the first threshold, the input port of the control cylinder is in fluid communication with the oil return port of the oil tank. an oil tank, comprising an oil supply port and an oil return port; a working pump, comprising a displacement changing mechanism for changing a displacement of the working pump, wherein an oil inlet of the working pump is in fluid communication with the oil supply port of the oil tank; a control cylinder, used for controlling the displacement changing mechanism; a working oil line, in fluid communication with an output main path of the working pump; a load-sensing system, comprising a load-sensing circuit which outputs hydraulic pressure based on a load; and a load-sensing valve and a fixed/variable-displacement switching control valve, wherein a first control port and a second control port of the load-sensing valve are respectively in fluid communication with the output main path and the fixed/variable-displacement switching control valve; both the working oil line and the load-sensing circuit are in fluid communication with the fixed/variable-displacement switching control valve; the load-sensing valve is in fluid communication with the output main path, the oil return port of the oil tank and an input port of the control cylinder; the fixed/variable-displacement switching control valve is switchable between a first valve position and a second valve position, when the fixed/variable-displacement switching control valve is in the first valve position, the load-sensing circuit is in fluid communication with the second control port of the load-sensing valve, when the fixed/variable-displacement switching control valve is in the second valve position, the working oil line is in fluid communication with the second control port of the load-sensing valve, to control the working pump to switch between a fixed displacement state and a fixed/variable displacement state; when a differential pressure between input pressure of the first control port and input pressure of the second control port is greater than a first threshold, the output main path is in fluid communication with the input port of the control cylinder; when the differential pressure between the input pressure of the first control port and the input pressure of the second control port is less than the first threshold, the input port of the control cylinder is in fluid communication with the oil return port of the oil tank.