看了您的回答更頭疼了，因為我每次都是在國內待五六個月後再在美國待五六個月。這樣的話我的服務期豈不是應該歸零。我還要在國內連續待兩年才行？
-Not need to be 國內連續待兩年, but it is aggregate of two years. That means you can visit US and stay in US for a certain time, not specific how long. So, you may be still OK to calculate your total time of stay in China for the home resident requirment. When it reaches total of two years, you may apply for the green card or H1.
What I commented above is that I am sure short visiting US (e.g.2 to 3 months) during the home resident should be OK, but it does not mean "每次都是在國內待五六個月後再在美國待五六個月" will "服務期應該歸零" (I am persionally just not sure it).
