The invention discloses an undrained shear strength indoor combined measuring instrument and an application method thereof. The undrained shear strength should be lower than 100 kPa. The measuring instrument mainly comprises an elevation pressurizing system, which is composed of a pressure servo motor, a transverse rotary rod, whose two ends are both provided with a gear, a rack, and an elevation rod of a piston pressure plate; a shearing system, which is composed of a rotation speed servo motor, an elevation rod of the rotation speed servo motor, and a cross-shaped shearing plate or round disc; and a measurement control system, which is composed of a torque sensor, a soil pressure box, PLC, and a touch screen. Same driving, elevating, measuring devices are adopted, only the cross-shaped shearing plate and round disc are changed directly, and the undrained shear strength of interface between soil and a steel plate can be measured. Relatively speaking, the equipment cost is low, and the relationship between the undrained shear strength of unsaturated soft soil and pressure/rotation speed can be researched.