In a closed four-level coherent atomic system,we theoretically studied the effect of pump field and regulated field intensity on signal gain.Studies have shown that in a circulating four-level coherent atomic medium,the regulated field changes the distribution of population among the four energy levels,causing the signal field gain to be amplified.When added the regulated field (the intensity of regulated field is equal to that of the signal field),the signal gain is improved 12 times in contrast with N-type four-level system.Coherent pump field induces signal field amplification,and changing the pump field intensity can achieve signal field from the corresponding population number inversion gain to no population inversion gain.As the intensity of the pump field increases,the value of signal gain grows and then reduces.The closed-loop system may have important applications in optical amplification and gain effect.