A pulse width modulation device for use in an N-ports random access memory having a plurality of word line sets, wherein a specified word line set comprises N port word lines. The pulse width modulation device comprises a status detecting device and a clock signal generator. The status detecting device is coupled to the N port word lines having a first and a second port word line, and outputs a first control signal when both the voltage values of the first and second port word lines are within a first level range. The clock signal generator is coupled to the status detecting device and the specified word line set, and generates and outputs a first clock signal to the specified word line set, wherein a duration of the first clock signal kept within the first level range is variable in response to the first control signal.