According to the prevention of support crushing during downtime of the fully mechanized caving face in Juye coal field, the survey of support working resistance was carried out during downtime, and support load time effect in the fully mechanized caving face with deep alluvium and its production mechanism were analyzed. The results show that: the support load during downtime has obvious time effect, and it experiences the three stages of rising, reaching peak and decreasing, which show the characteristic like roof weighting. The load time effect of supports on the solid side of working face is more obvious than that of supports on the gob side of working face, and the time effect of the front column load is more balanced than that of the back column load. The load reaching its peak of the support is not consistent with that of the other supports, but the peaks of supports are almost equal. The significant increasing of the back column working resistance is a precursor of roof weighting, and can be an index for roof weighting prediction. The production mechanism of support load time effect in the fully mechanized caving face with deep alluvium lies in: structure block of the lower rock beam rotates under the oppression of higher rock beam bearing the load form the alluvium, and it causes the increase of top coal convergence and support load. Furthermore, control measures of the support load time effect were put forward. the disaster of support crushing resulted from support load time effect can be avoided by selecting downtime chance, controlling downtime time and using the characteristic of “descending and unloading” of support pistons. The results can provide a basis for working face roof control design and roof safety control.