Considering the limitation of investment cost, a unified optimal placement method of voltage sag and harmonic resonance monitoring points is proposed. Firstly, the observability indexes of voltage sag and harmonic resonance are defined and quantified; Secondly, considering the observability of voltage sag and harmonic resonance as well as the cost economy of investment, the quantitative analysis method of the comprehensive performance of monitor placement programme is given based on analytic hierarchy process method and fuzzy comprehensive evaluation method; Then, a unified optimal monitoring points placement model of is proposed, which takes the highest comprehensive score of the monitor placement programme as the objective and the number of monitoring devices not exceeding the limit as the constraint, and is solved based on the discrete particle swarm optimization algorithm; Finally, the unified optimal placement of sag and resonance monitoring points is carried out based on IEEE30 bus test system. The simulation results show that the proposed method can obtain the optimal monitoring programme, whose voltage sag observability, harmonic resonance observability and economy of monitoring devices are coordinated optimal under the condition that the number of monitoring devices meets the investment cost limit..