Abstract: To clarify the effect of nitrogen fertilizer on the number of filled grains and grain weight of low- temperature wheat at the jointing stage，a pot experiment was carried out with wheat variety HC3366 as the material. Low- temperature stress treatments for 5 days（LT-T1） and 10 days （LT-T2） were carried out at the jointing stage，with natural temperature as control（CK-LT），and nitrogen fertilizer（LT-N）was applied on the end of low- temperature stress（LT-N）. The spike traits，spikelet development，and floret development of potted wheat were compared and analyzed. The results showed that compared with CK-LT，with the extension of low- temperature time，the decrease of spike number，grain number per spike，grain weight，total spikelet number，and total floret number under LT treatment gradually increased；the number of top-fruiting florets，basefruiting florets， strong-fruiting florets， and weak-fruiting florets all decreased significantly，and among them，the number of base-fruiting florets and weak-fruiting florets decreased the most. Compared with LT，with the extension of low- temperature time，the increase of grain number per spike，spike number，grain weight，total spikelet number，and total floret number under LT-N treatment gradually increased；the number of top-fruiting florets， base-fruiting florets， strong-fruiting florets， and weak-fruiting florets all increased，and among them，the number of base-fruiting florets and weakfruiting florets increased the most. In summary，low temperature at the jointing stage mainly reduced the number of base-fruiting florets and weak-fruiting florets，reduced the seed setting rate of florets，and then reduced the number of grains per spike and the weight of weak grains. Top-dressing nitrogen fertilizer after low- temperature mainly increased the number of base-fruiting florets and weak-fruiting florets，increased the seed setting rate of base-fruiting florets and weak-fruiting florets，and increased the contribution rate of base-fruiting florets and weak-fruiting florets to grain number per spike， thereby increasing grain number per spike and weak grain weight，and reducing yield loss caused by low- temperature stress.

Key words: wheat, low- temperature stress, nitrogen fertilizer, jointing stage, number of filled grains