Thermochromic　smart　windows　are　considered　to　be　promising　energy-saving　devices　for　reducing　energy　consumption　in　buildings.　The　ideal　materials　for　thermochromic　smart　windows　should　have　high　transmittance,　high　solar　modulation,　low　phase-transition　temperature,　and　excellent　high-temperature　thermal　stability,　which　are　difficult　to　achieve　simultaneously.　This　work　reports　a　simple　one-step　low-temperature　polymerization　method　to　prepare　a　thermo-responsive　poly(N-isopropylacrylamide)/hydroxypropylmethyl　cellulose　(PNIPAM/HPMC)　hydrogel　achieving　the　above　performances　simultaneously.　The　low-temperature　polymerization　environment　endowed　the　hydrogel　with　a　high　luminous　transmittance　(Tlum)　of　90.82%.　HPMC　as　a　functional　material　effectively　enhanced　the　mechanical　properties　and　thermal　stability　of　the　hydrogel.　Meanwhile,　the　PNIPAM/HPMC　hydrogel　showed　a　low　phase-transition　temperature　(∼32　°C)　and　high　solar　modulation　(ΔTsol　=　81.52%),　which　proved　that　it　is　an　ideal　material　for　thermochromic　smart　windows.　Moreover,　a　PNIPAM/HPMC　smart　window　exhibited　high　light　transmittance　(T380-760　=　86.27%),　excellent　light　modulation　(ΔT365　=　74.27%,　ΔT380-760　=　86.17%,　and　ΔT940　=　63.93%),　good　indoor　temperature　regulation　ability　and　stability,　which　indicated　that　it　was　an　attractive　candidate　for　application　in　reducing　energy　consumption　in　buildings.　This　work　also　provides　an　option　and　direction　for　modifying　PNIPAM-based　thermochromic　smart　windows.　©　2023　American　Chemical　Society.