This mixed methods study is aimed to examine the feasibility of integrating mathematical problem solving with architectural design via a 3D epistemic simulation game to promote active mathematics learning for middle-school students. The experimental-control pretest/posttest group design was adopted to examine whether the experience of interacting with an architecture simulation game would improve students' math knowledge for and performance of problem solving. Data were collected from 61 6th graders via both quantitative and qualitative methods, including math problem-solving and mental rotation tests, video- and screen-capture of game play behaviors, observation, as well as game logs. The study results indicated that the gaming group performed significantly better than the non-gaming control group in the math context problem solving test. The infield observation and participants' gaming behavior analysis suggested that the learning and practice of mathematical problem solving during gaming is a cognizant and planned endeavor framed by carefully designed game actions and objects.