The Â£16m Black Rock site in Portishead had to be partially demolished after the fire led to the roof's collapse. An anarchist group claimed responsibility.
It will be used by more than 300 officers per year from three police forces in the West Country.
Facilities include firing ranges, fake houses, interactive target systems and an abseil training area.
Police said it was due to be opened last year until it was extensively damaged in the "domestic extremism related arson attack" in August 2013, while in the final stages of construction.
Home Secretary Theresa May, who visited on Thursday, said the facility would "help maintain the very high standards that we expect of our firearms officers and equip them with the skills they need to protect the communities they serve".
"This is another example of how good police forces are collaborating with one another to make savings, pool resources and deliver more efficient local policing," she added.
A Tri Force collaboration has seen Avon and Somerset, Gloucestershire and Wiltshire Police forces share services including firearms, roads policing, collision investigation and dogs.
A police firearms training centre has been officially opened, two years after it was damaged in a major arson attack.