The invention discloses a coconut charcoal fiber flexibilizer for improving mechanical properties of well-cementing cement. The coconut charcoal fiber flexibilizer is prepared by the following steps: (1) uniformly mixing coconut charcoal fiber and dichloromethane, adding anhydrous ethanol, carrying out high-speed dispersion, performing suction filtration and drying; (2) adding the pretreated coconut charcoal fiber into deionized water, carrying out ultrasonic oscillation and stirring and dispersing, adding sodium dodecyl benzene sulfonate as an emulsifier, slowly dropwise adding methyl methacrylate and an initiator potassium persulfate, introducing nitrogen for protection, heating to 85 DEG C, performing thermostatic reaction for 8 h, cooling, aging for 2-3 h, carrying out centrifugation sedimeantation on a product, filtering, washing, carrying out suction filtration, and drying under reduced pressure; and (3) placing emulsion polymerization modified coconut charcoal fiber into spray-drying equipment and drying at 100-200 DEG C for 30 min. According to the invention, original performance of coconut charcoal fiber is maintained, and coconut charcoal fiber is greatly combined with a cement material. Thus, brittle structure of cement is changed to some extent, and mechanical properties of well-cementing cement can be improved.