Some cracks occurred on arch of an opencut doubletrack railway tunnel. Hence, the causes of cracks, their distribution and morphologies are analyzed. It is revealed that the width of arch crack is positively correlates to thickness of backfill rock; the width of arch crack ranges from 0.55 mm to 1.00 mm when the thickness of the backfill rock reaches 15 m. The results of numerical analysis of opencut tunnel construction show that: (1) When the thickness of the backfill rock is less than 5 m, the arch tensile stress increases with the thickness of the crown backfill rock decrease； and meanwhile the construction dynamic load has a greater impact on the structural stress of the opencut tunnel. (2) The arch tensile stress is significantly affected by the backfill compactness of sidewalls;when the compactness is up to 60%, the arch tensile stress is 1.44 times that of when reaches complete compactness, which beyond the ultimate strength of tunnel structure. The comprehensive analysis shows that the premature backfill construction and the uncompacted backfill are the main causes of cracks on tunnel arch. It is proposed that the construction dynamic load shall be incorporated into the design calculation of the opencut tunnel and the timing of backfill shall be strictly controlled during construction. In addition, the construction management measures shall be enhanced and the study results can provide reference for similar projects in the future.