The flow monitoring is one of the important parts for grouting in bed-separation of overburden.By making use of the optical grating measuring technology,the concentration determination of liquid-solid two-phase flow has been realized.And by programming,Programmable Logic Controller is used to control the whole process of flow monitoring.The estimated error of the grating measuring system is between-0.083%~ 0.248%.The grating measuring system can reduce the cost of flow monitoring and achieve the automation of monitoring process for grouting with fly ash.