Although the geopolymer is generally regarded as a solution to low-carbon building materials, the intrinsic high brittleness greatly limits its further engineering practices. Here, we performed multi-scale ab-initio modeling and experiment on synergistic effect of nano- calcium carbonate (NCC) and polyethylene fiber (PEF) on toughening mechanisms of geopolymer composites for the first time. Results showed that NCC significantly improved micro-scale geopolymer gels aggregation on the surface of PEF through the nano-scale calcium (Ca)-triggered electrostatic attractions and high bonding between N/CASH-polyethylene. Due to micro-scale gel aggregation, meso fiber dispersion, geopolymers with 12 mm + 6 mm PE fiber + 1% NCC showed the best synergistic effect on macro- scale bending toughness and multiple cracking characteristics. The relationship of fiber reinforcing index-toughness, toughness-crack amount and toughness-crack fractal dimension tracked good linear, natural log and exponential function, respectively. The crack number and crack fractal dimension are applicable visible indicators of the bending toughness of PEF reinforced GCs.