Horizontal well greatly propels the development of unconventional oil and gas. Considering the drilling safety and basic demand of hole cleaning, define length of horizontal well as extreme hydraulic extension length when the drilling fluid pump rate is equal to minimum cutting-carry pump rate and hole cleaning satisfied basic demand. Based on this concept, a prediction model is established according to the relationship among minimum cutting-carry pump rate, bottomhole pressure, circulation pressure loss and drilling pump pressure. This model considers the influence of cuttings on the hydrostatic column pressure, horizontal annular pressure drop and annular geometrical shape. And then this model is used to analyze the impact of well structure, formation property, drilling fluid property, hole cleaning degree and drilling parameters on extreme hydraulic extension length. On the above basis, a new index called ration of dispersion coefficient is introduced to evaluate the sensitivity of each factor. The sensitivity decreases as the sequence of cutting particle size, drilling fluid flow behavior index, acceptable cutting bed height, wellbore diameter, ROP(8-15 m/h), drilling fluid density, drilling string eccentricity, formation fracture pressure, drilling fluid consistency index, well vertical depth and ROP(1-8 m/h). Based on both mitigation effect and feasibility, cutting particle size, drilling fluid flow behavior index, acceptable cutting bed height and rate of penetration are worthy to optimize to prolong EHEL.