Recently, ghost imaging with thermal light attracts a lot of attention both theoretically and experimentally. The effects of the detector size and defocusing length on lensless ghost diffraction are investigated. Through analyzing the intensity correlation function of lensless ghost diffraction, the defocusing will lead to a chirp modulation on the phase of the transmission function of the object, while the finite size of the detector makes the lensless ghost diffraction system equivalent to a partially coherent imaging system. These effects lead to the degradation of the imaging quality. Via calculating the correlation of the idealized images and realistic images under different conditions, the dependences of diffraction qualitites on different parameters are quantitatively analysed, so that the results may be used to provide quantitative references for realistic experiments.