The invention discloses a method for measuring the viscosity of thick oil. The method comprises the following steps: heating a high pressure falling ball viscometer to a formation temperature, maintaining the formation temperature for more than 4 hours, evacuating the viscometer; fully filling a pressurized pipeline, the high pressure falling ball viscometer, and a sample transfer pipeline with a saturated zinc sulfate solution; lifting the pressure of the high pressure falling ball viscometer to the formation pressure value; communicating a sample storage device with the sample transfer pipeline; maintaining the formation pressure, continuously reducing the volume of a high pressure metering pump (A) to pump a crude oil sample into the high pressure falling ball viscometer to replace and discharge the saturated zinc sulfate solution in the high pressure falling ball viscometer, after all zinc sulfate solution is discharged, keeping on pumping 20 mL of crude oil sample into the high pressure falling ball viscometer; and measuring the ball falling time according to a measuring rule. According to the method, a mode of displacement of fluids with different properties is adopted to realize the stratum thick oil fluid balanced sample transfer so as to ensure that during the thick oil sample transfer process, the state and properties of the fluid are not changed, thus the uniqueness and stability of fluid tests can be guaranteed, and the analysis results are real and reliable.