The work aims to explore the mass fraction of HPMC with the best mechanical properties, with konjac glucomannan (KGM) as the matrix and hydroxypropyl methyl cellulose (HPMC) added to modify water-soluble packaging film. The HPMC and KGM were added into deionized water to prepare the blended solution successively, and then the film was prepared by casting method. The mass fraction of HPMC was adjusted, and the influence on its structure and properties was tested and analyzed. With the increase of HPMC mass fraction, the tensile strength and elongation at break of the films first increased and then decreased, and the maximum value existed. However, the water solubility of the films tended to decrease gradually and the decrease of the water solubility was still small, which could still meet the needs of the water-soluble packaging; the transmittance gradually increased and the haze gradually reduced, which was conducive to the application in packaging. For the film-forming liquid (mass fraction: 1%) of HPMC and KGM, the mechanical properties of the film are better when the mass fraction of HPMC is 20% of the total.