Changes in the proportions of male and female flowers in monoecious plants in response to external environmental conditions are directly related to the reproductive fitness of plants. The monoecious cucumber (Cucumber sativus) plant was used in this study to assess the responses of sex differentiation and the breeding process to nutrient supply and the degree of artificial pollination using pollen solutions of different concentrations. We found that the nutrient supply significantly improved the number of female flowers, while pollination treatments did not obviously increase the number of male flowers. Continuous pollination changed the number of female flowers especially in the later stage of the pollination experiment. Therefore, pollination changed the ratio of male and female flowers in the flowering stage of cucumber. Pollination treatment affected the fruit growth, seed set, and fruit yield. The number of fruit, fruit set percentage, and total seeds per plant did not increase with the pollination level, but individual fruit weight and seed number in one fruit did increase. The differentiation of male and female flowers in the flowering stage of cucumber is a response to nutrient and pollination resources, but this response is not the optimal resource allocation for subsequent fruit development and seed maturity, which suggests that the response of plants to external environment resources is short-term and direct.