The proposal is included in Congress' bipartisan budget deal, which President Barack Obama signed last week.
The cut amounts to a 1-percentage-point reduction in the annual cost-of-living increase for working-age military retirees.
Lt. Col. Stephen Preston, a 25-year-veteran who served in Afghanistan and Iraq, said he was enraged by the proposal, the Washington Post reported.
Preston, 51, collects a pension of nearly $55,000 a year, which allowed him to go back to college, get an MBA and begin a second career.
"I'm not an angry man, but I was very, very angry," Preston told the Washington Post. "If you didn't want to pay us what you promised us, then you probably shouldn't have promised it."
Ryan has argued that the cuts are necessary.
"I stand behind the need for reform," he wrote in a Dec. 22 op-ed in USA Today.
Military compensation, including health benefits and salaries, account for half of the defense budget.
Military members who serve for 20 years can retire and receive annual cost-of-living adjustments, meaning that those who sign up at age 18 can start receiving a full pension at 38.