Wildfire often causes the loss of lives, damage to property, and alteration of the environment. Great efforts have been made in the world to reduce the impacts of fire including fuel reduction and modification, prescribe burning, and firebreak construction. Firebreaks have been used in forest fire management in China for a long time; however, no assessments of their effectiveness in fire risk reduction due to density, width, and spatial pattern have been conducted. In this study, we used burn probability (BP) and construction cost as measures to determine the optimal firebreak design for the boreal forests in Northeast China. Using the current landscape without artificial firebreaks as the baseline, we assessed the BP changes for 49 simulated firebreak scenarios that included combinations of eleven densities, three widths, and two spatial patterns. The Burn-P3 model was used to simulate BP for each scenario based on weather conditions from fire seasons during 2005–2018. Our results showed that: 1) BP decreased with increasing firebreak density when width and spatial pattern remained constant; 2) BP decreased significantly within 2000 m of firebreaks when width increased from 30 m to 60/90 m and remained at a constant density; 3) areas of high and very high BP can effectively be reduced through the use of spatially optimized firebreak designs; and 4) an optimal firebreak design was determined that is 4.1 m/ha in density, 30 m wide in grasslands and 60 m wide in forests, and in the optimized BP-based spatial distribution pattern. The optimal firebreak design may reduce BP by 25.2% in comparison to that under current scenario, with further reductions in high- and very-high-BP areas.