The onset process of a thermoacoustic prime mover is a process generating and maintaining self-excited oscillation. It is helpful to have a better understanding of thermoacoustic effect by investigating the mechanism of thermoacoustic self-excited oscillation. The network model of a representative standing-wave thermoacoustic prime mover is established on the basis of thermoacoustic network theory. Comparing thermoacoustic network to electric network, the apparent power flux which inputs the thermoacoustic network is calculated by using the Hermitian form. In the network, the apparent power flux balance means establishing the self-excited oscillation. Based on the above, the threshold temperature and operation frequency of a thermoacoustic prime mover are calculated on condition that the imaginary part of angular frequency is equal to zero. The calculation results are in good agreement with the experimental results. For the coupling relationship of the main pressure with the threshold temperature and operation frequency, the calculation results are roughly close to the experimental results. The obtained results are helpful for the further studying of the thermoacoustic effect and the optimal designing of a thermoacoustic system.