Hydroxypropyl amylomaize starch (HPAS) films were prepared by hot press. The effects of initial pH of HPAS on the mechanical properties, molecular interaction, structure, and cross-linking degree of the resultant films were investigated. A weak acidic condition was suitable for cross-linking of citric acid and HPAS by reactive extrusion. The film of HPAS with an initial pH of 5.66 had the maximum tensile strength of 7.20 MPa and elongation-at-break of 94.53%, and the weight average molecular weight of HPAS increased to 4.17 × 105 g/mol. An appropriate initial pH facilitated the formation of diester bonds between HPAS and citric acid during extrusion, but too low initial pH levels resulted in hydrolysis of starch molecules and reduced the mechanical properties.