Abstract: To investigate the influence of polyethylene (PE) fiber length on the macroscopic mechanical properties and crack evolution mechanisms of engineered geopolymer composites (EGC), compressive strength and uniaxial tensile tests were conducted on fly ash–based and slag-based EGC with varying fiber lengths. Crack evolution parameters during the tensile process were systematically collected. A probabilistic model for crack width evolution was established based on the two-parameter Weibull distribution. Fractal theory was employed to characterize the morphology and propagation behavior of multiple cracking, and a correlation model between fractal dimension and tensile performance was subsequently developed. The results indicate that fiber length has a limited effect on the compressive strength of the matrix, with the cube compressive strength of fly ash–based and slag-based EGC concentrated around 70 MPa and 120 MPa, respectively. In terms of tensile behavior, when the fiber length exceeds 9 mm, EGC exhibits robust multiple cracking and pronounced strain-hardening behavior. Specimens with fiber lengths of 18 mm and 24 mm show typical over-saturated multiple cracking characteristics accompanied by a two-stage strain-hardening response. The crack width evolution probability model based on the two-parameter Weibull distribution accurately captures the over-saturated multiple cracking behavior of EGC. Further fractal analysis using the box-counting method reveals that the crack patterns of all specimens exhibit clear fractal characteristics, with fractal dimensions ranging from 1.1020 to 1.3672. Moreover, the fractal dimension shows significant linear correlations with crack parameters and tensile strain capacity, and the proposed mathematical model demonstrates good predictive capability for engineering applications.