In the weak segregation limit,the structure evolution of the hexagonal cylindrical phase of diblock copolymers in films was investigated.Employing the Landau-Brazovskii mean field theory,we obtained three amplitude parameters as functions of temperature,surface field strength and film thickness.By controlling confinement size and surface field strength,lamellae and undulated lamellae appear in the cylindrical bulk phase of diblock copolymers."Phase diagrams" of confinement-induced structures are constructed at different surface field strengths.The obtained theoretical results are in agreement with relevant theoretical and experimental results.