Thermal stability, chemical structure and microstructure of cellulose fibre under cement paste and dry conditions at different temperature were tested to investigate its temperature adaptability in cement-based composites. Results demonstrated that after curing for 60 days at 60 and 90 °C, the compressive strength of cement paste containing 0.4 wt% cellulose fibre increased 16.16% and 22.79%, respectively. However, the compressive strength of the paste curing at 120 and 150 °C had no enhancement effect. Compared with the dry condition, cement paste filtrate had a significant influence on chemical structure and microstructure at 120 and 150 °C. In filtrate cured at 120 °C, β-1,4-polyglucose in cellulose fibre underwent alkaline hydrolysis, fragmenting cellulose fibre to smaller pieces, which converted the “fibrous structure” to “cluster structure”. However, in filtrate cured at 150 °C, reducing end group in cellulose fibre underwent peeling reaction, which decreased its thermal stability and changed its crystallinity from 0.69 to 0, whereby microstructure transformed into “agglomeration structure”.