The watermelon strain W117 was treated with ethylmethanesulfonate (EMS) at 1.0% for 9 h, and phenotypic variation of M₁ and M₂,including the leaf, flower, stalk, fertility and branching habits etc, were studied. The typical mutant lines of M₂ were selected simultaneously, and 23 watermelon SSR primers were used for analysis and identification, and the mutant library of watermelon was constructed finally. The results showed that: (1) the M₁ seedlings mutagenized showed differently morphological characteristics such as leaf deformity, leaf folds, partial yellowing, floral abnormality, male flowers do not loose powder, tendril deformity, short stature, slow growth and infertility, ect. The M₁ population of watermelon mutant was composed of 1 252 individual plants, the total mutation frequency was 18.33%.(2) 205 M₂ mutant plants were screened, and 40 phenotypic variations were found in cotyledon traits (yellowing, asymmetric twist, fold etc.), leaf and stem traits (leaf yellowing, smaller, cracked deep, thin stems, internodes shorter, less branched and so on), floral traits (flowers become larger, lighter color, bisexual flowers, petals shrinkage, partial degradation, the number of mutations, stigma deformities, stamens immature) ,and other traits (slow growth, infertility, etc.), the total phenotypic mutation rate reached 19.59%. (3) For the 10 representative mutant plants of M₂ generation, 9 samples were found to be mutated at the DNA level by SSR primer analysis. In this study, a mutant library of watermelon containing 120 M₁ generation lines and 1051 M₂ generation plants and 40 phenotypic variations was constructed.