A viscometer based on an ultrasonic-coupled tilted fiber grating is disclosed. The viscometer includes a viscometer body, an optical signal modulating and demodulating module, an ultrasonic wave signal generating module and a processing and displaying module. When the viscometer is used in measurement, the tilted fiber grating viscometer body is put into a liquid to be measured, and a reflection spectrum output having a narrow linewidth is obtained by utilizing grating coupling generated by the tilted fiber grating and ultrasonic waves. By utilizing a damping effect of viscosity on ultrasonic wave vibration and by detecting wavelength and intensity changes of the reflection spectrum coupling, the viscosity of a material is obtained by back calculation. A temperature sensing module is utilized to compensate influences of temperature changes on viscosity measurement. A film layer having a low refractive index is utilized to separate the tilted fiber grating from contact with the outside, thus avoiding viscosity measurement inaccuracy caused by influences of material refractive index changes on spectrum responses.