The invention discloses a sexual polyploidization breeding method for tetraploid petunias. The method comprises the following steps: firstly, determining the sizes and the morphological characteristics of flower buds at a reduction mitosis prophase I of diploid petunias to grasp the optimal treatment period; carrying out inducing treatment with colchicines with optimal concentration for proper times, observing the sizes and the morphological characteristics of petunia 2n pollen and n pollen of the petunias to rapidly distinguish and identify the 2n pollen; carrying out sexual polyploidization hybridizing; and identifying ploidies of hybridized F1 plant of the petunias by a simple and easy cytology identification method. With the adoption of the breeding method, the obtained sexual polyploidization hybridized F1 not only has the characteristics of large volume of a parent polyploidy organ, but also has the inheritable characters from a diploid male parent; compared with an autotetraploid obtained by an asexual polyploidization manner, the tetraploid petunias have wider genetic diversity, and higher ornamental value and seed setting rate.