The plant mutation effects induced by ionizing radiation involve a rather complex process which is composed of physical, chemical, biochemical and biological stages. Nowadays, although ionizing radiation has been widely used in plant mutation breeding, the theoretical explanations for the mechanism of the ionizing radiation caused plant mutation effects are insufficient. Especially, a saddle shape relationship between the plant survival rate and radiation dose is found in the mutagenis effect of ionizing radiation on plants. The underlying mechanism of the saddle shape relationship remains unclear and challenges to all extant models.To explain this relationship, a damage-repair model for the plant mutation effects induced by ionizing radiation is proposed in the present work. Our model is based on the rate theory of ionizing radiation in which the cell damage and repair are taken into account simultaneously together with the micro-and macro-biological mutation effects of plant caused by ionizing radiation. The states of the radiated plant individuals are grouped into three categories: normal, damaged and lethal categories in our model. The evolution dynamics of the relative concentrations of the three categories are determined by a set of coupled equations which are mathematically the same as the Crow-Kimura equations in species evolution theories. With the numerical solution of our model in its steady state, the relative steady state concentration distributions of different categories of the radiated plants with increasing radiation dose are obtained. It is shown that without the plant repair effect, the relationship between the plant survival rate and radiation dose appears to be a conventional shoulder type one. With the plant repair effect, our model gives a saddle shape survival-dose relationship which has been observed commonly in the experiments on the radiated plants by ionizing radiation. To further test the model, the experimental data on the inbred lines of maizes radiated by heavy ion 7Li are used to determine the parameters of the model. It is shown that the theoretical results are basically consistent with the experimental ones. In addition, the mutation characteristic of the survival plants also appears to be a saddle effect-dose relationship, for which the theoretical model could also give a reasonable explanation. Our damage-repair model explains the saddle shape relationship between the plant survival rate and radiation dose, which indeed illuminates its power. And it provides a theoretical basis and reference for studying the biological effect mechanism of plants induced by ionizing radiation and conducting ionizing radiation plant breeding.