Small　and　medium　span　bridges　account　for　86%　of　the　bridges　in　China,　and　maintenance　is　necessary　to　ensure　their　safety　and　serviceability.　The　infrastructure　sector　causes　substantial　CO2　emissions,　and　it　is　one　of　the　most　polluting　sectors.　Fly　ash　(FA)　is　often　used　in　bridges　to　reduce　CO2　emissions.　Moreover,　the　lack　of　temporal　information　is　an　important　limitation　of　the　life　cycle　CO2　assessment　of　bridges.　Taking　the　upper　structure　of　small　and　medium　span　bridges　as　the　research　object,　this　paper　proposes　a　Monte　Carlo　simulation-based　life　cycle　multi-objective　dynamic　maintenance　strategy　optimization　model　to　analyze　the　life-cycle　carbon　emissions,　costs,　and　life　cycle　reliability　of　different　preventive　maintenance　(PM)　and　essential　maintenance　(EM)　methods　of　FA　reinforced　concrete　(RC)　beams.　Considering　the　continuous　effect　of　carbon　emissions　and　the　net　present　value　of　the　cost,　the　time　effect　of　carbon　emissions　and　costs　of　maintenance　and　reinforcement　at　different　times　is　analyzed.　Through　50,000　simulations　of　the　case　beam,　the　parameter　characteristics　of　different　optimal　reinforcement　schemes　and　the　relationship　between　the　reliability,　life　cycle　CO2　emissions,　and　costs　are　studied.　When　introducing　the　discount　rate,　the　essential　maintenance　with　higher　carbon　emissions　and　costs　has　a　marginal　effect,　i.e.,　the　longer　that　future　carbon　emissions　and　costs　occur,　the　smaller　the　net　present　value.　The　proposed　method　is　demonstrated　to　be　effective　at　supporting　decision　making　for　bridge　managers.　It　would　be　helpful　for　identifying　the　effectiveness　of　PM　and　EM,　selecting　optimal　maintenance　time　interval,　and　providing　a　rational　estimate　of　the　CO2　emissions　and　costs　of　bridges.