In ovo administration of IGF-1 to poultry eggs has effective roles on post hatching muscle development. However, the secondary muscle development stages at the late embryo development stage are important for muscle fiber formation and differentiation. To investigate the roles of in ovo administration of IGF-1 on duck secondary muscle development, we injected rhIGF-1 into duck eggs in hatching at day 12. After administration on days 18, 21, 24, and 27 in hatching (E18d, E21d, E24d, and E27d, respectively), muscle samples were isolated, and the muscle tissue weight, muscle fiber parameters, and myoblast proliferation rate in leg and breast muscle were analyzed. Additionally, the expression levels of the transcription factors MyoG and MRF4 were detected using qPCR. Results show that embryo body weight and muscle fiber parameters, including muscle fiber diameter (MFD) and the number of myofibers per unit area, are upregulated in IGF-1-treated groups. Moreover, the transcription factors MyoG and MRF4 are expressed at higher levels in the experimental groups compared with the control groups. These results suggest that in ovo administration of IGF-1 to poultry eggs can mediate the expression of MyoG and MRF4, induce myoblast proliferation, and finally influence muscle development during the secondary muscle development stages.