ACE was launched on August 15, 1997 and placed into an orbit about the L1 point between the Earth and the Sun. The L1 point is one of several points where the gravitational attraction of the Sun and Earth are equal and in the opposite direction. The Parker spiral angle at L1 point with ACE MAG data is calculated and the data is converted to the polarity symbolic sequence. For any point in the sequence, the quasi 27-day reoccurrence period and the structure of IMF are detected through altering the sub-sequence centered on this point. And then, the reoccurrence structure is identified with the template matching method. Finally, the piecewise constant function fitting is used to obtain the piecewise constant period and structure. The obtained results are as follows: (1) in the period of 1999—2008, the reoccurrence period is mostly in the range of (27±2) days, with only one exception: a month from 2001-10-27 10:00 to 2001-11-28 12:00, the reoccurrence period is 21 days and 14 hours; (2) from 2001-01-08 12:00 to 2004-07-17 04:00, IMF is in a two fan disc structure; from 2004-07-17 05:00 to 2007-06-30 23:00, IMF is in a four fan disc structure; from 2007-07-01 00:00 to 2008-12-31 23:00, IMF is in a two fan disc structure.