[Objectives]  To determine the effect of nest tube diameter on the population dynamics of Osmia excavata Alfken, an important pollinator of apple crops in northern China. [Methods]  Bee weight, cocoon length, sex ratio, parasitism rate and mortality, of colonies nesting in tubes of three different internal diameters (6, 8 and 10 mm) were measured and compared by placing nest tubes of these diameters in apple orchards. [Results]  (1) O. excavata has marked sexual size dimorphism; adult females were 1.67 times heavier than males, and the length of female cocoons was 1.15 times that of male cocoons. (2) Increasing the internal diameter of the nest tube within a certain range can significantly increase the number of female progeny, and cocoon weight and length. (3) Parental investment of O. excavata increased with increasing nest tube diameter. [Conclusion]  Nest tube diameter affects the parental investment and sex ratio of O. excavata. In smaller diameter nest tubes, offspring tend to be male, whereas in larger diameter nest tubes offspring tend to be female. The available tube diameter can therefore affect bee weight, cocoon length and the sex ratio of progeny, and thereby the structure and stability of the local population. Changing the size of nest tubes can affect the size and sex ratio of offspring, providing a relatively simple way to optimize the management of this semi-domesticated, solitary bee species. The best nest tubes had an internal diameter in the range of 8-10 mm.