The ultra-heavy oil reservoir in Tahe Oilfield is currently producing by mixing light oils with ultra-heavy oils in wellbore to reduce viscosity. However, the high ratio of needed light oils and abnormal fluid production caused by uneven mixing make the production low efficient and high cost. To solve this challenge, in this work, we proposed the technology of catalytic ex-situ upgrading for viscosity reduction + reinjection of upgraded oil into wellbore to save the amount of injected light oils and improve the recovery efficiency. To develop a catalyst system that can effectively reduce the viscosity of the ultra-heavy oil and achieve high-level ex-situ upgrading, a Ni-Mo catalyst with high porosity based on γ-Al2O3 carrier was developed and its catalytic effect on the ex-situ upgrading and viscosity reduction was evaluated. The results show that the developed Ni-Mo catalyst can reduce the viscosity of ultra-heavy oil at 50 ℃ from 28200 mPa.s to 298 mPa.s. The viscosity is reduced by nearly 100 times, and the viscosity reduction rate is 98.94%. The density is decreased from 1.007 g/ml to 0.8724 g/ml. At the same time, the content of saturates is significantly increased, and the content of resins and asphaltenes was greatly decreased. The developed catalyst system shows an excellent ex-situ catalytic upgrading effect for extra-heavy oil and has a great potential for field application.