The invention provides an online viscosity detection device and method for a high temperature and high pressure oil-water mixed fluid. The online viscosity detection device for the high temperature and high pressure oil-water mixed fluid comprises a circulating mechanism, a temperature control mechanism and a data collecting and data processing mechanism. Various mature and high-accuracy instruments are mounted on the online viscosity detection device for the high temperature and high pressure oil-water mixed fluid and can continuously detect the temperature, pressure and pressure difference of the mixed fluid in a vertical pipeline in real time; and a computer collects data in real time and calculates the current viscosity of the fluid through a model pre-set in the computer. The online viscosity detection device for the high temperature and high pressure oil-water mixed fluid, provided by the invention, can measure the pipe flow viscosity of the mixed fluid under a high temperature and high pressure condition in real time, is suitable for single-phase and multi-phase fluids, has the advantages of less equipment investment, low cost, simple and convenient detection method and accurate and reliable detection and achieves wide engineering application values and academic values.