The invention discloses a rotational viscometer with a blade-shape motor, and a method of measuring particle fluid viscosity by the rotational viscometer. The rotational viscometer comprises a torque transducer, a support, a hoisting mechanism, a rotating rod, a swivel, a sample chamber, a blade-shape rotor, an experiment platform, a signal line and a control mechanism, wherein the blade-shape motor is fixed to the swivel, the swivel is connected to the torque transducer through the rotating rod; the torque transducer is disposed in the support of the hoisting mechanism, and the torque transducer and the hoisting mechanism are controlled by the control mechanism. A using method includes that the torque transducer rotates and drives the blade-shape motor to rotate, the blade-shape motor rotates in a particle fluid and is subjected to shearing stress of the particle fluid, torque changes sensed by the torque transducer are transferred to the control mechanism through electric signals, and the viscosity of the particle fluid can be calculated by the measured torque. The rotational viscometer and the method are advantaged by capability of accurately measuring the viscosity of various fluids at room temperature, high precision, good repeatability, high measuring speed, simple structure, low cost, long service lifetime and high production efficiency.