An algorithm for phasor measurement unit(PMU) configuration is presented to prevent the wide-area measurement system(WAMS) from losing its observability due to high risk cascading failures. Cascading failures are analyzed using risk theory and hidden failure model to identify high risk ones. And WAMS observability for single high risk cascading failure is strengthened by optimal configuration of PMU. Experimental results on IEEE39 system justify that the method is economic and robust.