I am glad to tell those gays who happen to meet the same question,that I have got it!
IRasterRGBRenderer2 m_pRasterRGBRenderer = pRasterLayer.Renderer as IRasterRGBRenderer2;
IRasterStretch2 pRasterStretch2 = m_pRasterRGBRenderer as IRasterStretch2;
IArray pArray = pRasterStretch2.SpecificationHistogram;
IArray pArray2 = pRasterStretch2.StretchStats;
object obj1 = pArray.get_Element(1);
object obj2 = pArray2.get_Element(1);
IStatsHistogram pStatsHistogram = obj2 as IStatsHistogram;
That's the answer! It'a IStatsHistogram.