The composition of Tahe ultra-heavy oil was analyzed by low field nuclear magnetic resonance（LF-NMR）. Based on the composition of Tahe ultra-heavy oil，four oil-soluble viscosity reducers based on aliphatic，aromatic，aromatic + aliphatic，polar compounds were developed；and their viscosity-reducing effect was comprehensively evaluated by LF-NMR，viscosity analysis， and immersion dilution. The analysis results of crude oil group components（SARA）verified the accuracy of LF-NMR technology in analyzing crude oil composition. Except the polar compound viscosity reducer，other viscosity reducers could reduce the viscosity of ultra-heavy oil by more than 90%. The viscosity reducing performance was ranked by aromatic ＞ mixture of aliphatic and aromatic ＞ aliphatic ＞ polar compound. Aromatic viscosity reducer had the best viscosity reducing effect and could reduce the viscosity from 61 000 000 mPa·s（50 ℃）to 14 897 mPa·s. At the same time，it could dissolve asphaltene and prevented asphaltenes from agglomerating. The experimental results proved that LF-NMR could not only be used as a fast and convenient method to analyze the composition of crude oils，but also be used to evaluate the effect of viscosity reducers，as well as to analyze the mechanism of viscosity reduction.