For　exposed　combustion　environments,　multi-view　optical　imaging　method　is　commonly　used　for　tomographic　radiation　thermometry.　However,　in　the　case　of　closed　combustion　with　limited　optical　testable　space,　it　is　ineffective　due　to　the　light　obstruction　by　burner　wall.　To　overcome　this　problem,　this　paper　proposes　a　monocular　multi-focal　imager　equipped　with　tomography　channel.　There　exists　difference　in　defocus　among　different　imaging　channels.　In　theory,　the　combustion　flame　is　segmented　in　the　form　of　parallel　slices　along　the　main　optical　axis　of　imager.　Based　on　the　Fourier　optics,　the　radiation　information　of　slices　at　different　spatial　axial　positions　is　extracted　by　establishing　the　mapping　relationship　between　object　and　image.　Then,　the　temperature　of　combustion　slice　is　further　calculated　utilizing　the　Planck　law.　In　experiment,　the　monocular　multi-focal　imager　is　designed　and　processed　by　optoelectromechanical　integration.　Multiple　imaging　channels　with　different　defocus　are　operated　with　common　optical　path　structure　and　synchronous　trigging　to　match　the　transient　three-dimensional　combustion　flame.　The　measured　results　show　that　the　temperature　distribution　of　flame　can　be　effectively　revealed,　which　demonstrates　the　potential　applicability　of　combustion　diagnosis.