According to The Sun, the Moneyball actor is "terrified" that Jolie will die in her special envoy role which sees her visit war-torn countries like Iraq and Syria.
"Brad's terrified something bad will happen to her. It's why he wanted to cement things with a wedding," an insider is quoted as saying.
The UN is set to launch a fresh offensive against South American drug lords.
The couple, who have been together since 2008, confirmed their engagement last month.
Jolie, 36, has previously been honoured by the UN for her humanitarian work.