The imaging quality of X-ray in-line outline imaging(XIOI)is mainly related to the size of samples, X-ray photon energy, X-ray scattering, distance from sample to detector Dsd and also distance from source to sample for point X-ray source and so on. To maximize the resolution of the image under the practical limitations of laboratorial equipment in distance from sample to detector and range of adjustable photon energy etc., the present paper studies the effect of the above factors on the imaging quality using numeral simulation. We obtained the relationship between imaging quality and photon energy, and not only found that there are certain X-ray photon energies which are equally well suitable for imaging but also found that we can get some high quality images for phase retrieval through which we can get the exact phase and absorption information. We have determined the precice Dsd values at which the imaging quality is the best for samples of different size, and found that Dsd is propertional to the sample size and determined the factor of proportionality. Though the diffuse scattering of X-ray reduces the contrast of images, we can still have good contrast when the variation in thickness is the order of the sample size when the scattering are only caused by the variation in thickness. So under these unfavorable conditions XIOI still performs effectively.