In order to obtain sufficient information about object-wave fields when there are large differences between the size of the object and CCD array, it is a common way to transform object light with lens system in the detection research of color digital holography. Based on the similar characteristics of object and image, the image of object is often viewed as object fields in image space and the reconstructed color digital image. It is a problem that must be solved to determine imaging plane of object when illuminated by lights with different wavelengths and to reconstruct these image fields with the same size. A new simple method of determining image plane of different-color lights without knowing lens parameters of optical system is presented. Further more, based on focal-depth research of digital holographic reconstructed image,the color image of object is reconstructed well in image space.