我自己想明白了，因为两种方法其实估计结果是相同的，只是用的baseline year不一样，所以截距项不一样，year dummy的系数不一样。btw,怎么删帖啊

The first command creates a series of dummy based on year. E.g. '_Ifyear_1972' treat observations in 1972 as 1 while the rest is valued in 0. Then you got group dummies for the regression and include year FE only. The second command uses the time series, i.e. one dummy for each year. And you use both year and firm FE.

Hope it's helpful to you.

I think both commands included both year and firm FE. They are exactly the same.