Abstract: With large-scale grid-connection of such new energy as wind power and so on, the subsynchronous oscillation will be even more prominent in the future, so it is urgently need to establish a new-type of subsynchronous oscillation monitoring system. For this reason, relevant research on the subsynchronous oscillation monitoring device, i.e., the subsynchronous phasor measurement unit (abbr. SPMU), has been conducted by Chinese scholars, and on this basis, how to economically and efficiently enact a configuration plan of SPMU becomes the first question in the construction of subsynchronous oscillation monitoring system. Based on the frequency-coupled impedance model of wind turbine units, a monitoring critical degree evaluation system, in which the differences among importance of evaluation indices and the probability of occurrence of oscillation under operating condition were taken into account, was proposed. Considering the situation that during the installation and configuration of SPMU there were often the multi-stage installation, a multi-stage optimal configuration model for SPMU, in which the nodal monitoring critical degree was taken into account, was constructed. Finally, the integral linear programming was utilized to solve the multi-stage optimal configuration scheme. Both accuracy and economy of the proposed indices and algorithm are verified by simulation results of ERCOT system in Texas, USA, and the modified New England 39 bus system.