During weaving, broken picks and irregular pick always lead to loom stop and cloth defect. Weft tension is one of the important factors. This paper introduces a kind of method that can be used to measure weft tension when the weft is in the shed on the loom. The weft tension in shed of 296.5 dtex + 70 D spandex covering combed cotton yarn was be measured on the TaiTan TT-800 air-jet loom, and the results we got is sound. It is seen that the method we used effectively solves testing weft tension in shed on air-jet looms.