The threshold of parametric inst abilities will be increased and the growth rate will fall down if the coherence time of pump laser is less than the rise time of parametric instabilities. The paper shows the finitebandwidth effect of reduction in parametric instabilitie s and superathermal electrons using chirped stretched laser pulse. Both analytical solutions and the results of numerical simulation using PIC code are presented.The effect of electronion collision on suprathermal electrons is also investigated.