Diffractive telescopes are one of the most promising solutions to build lightweight space telescopes with a diameter over 10 m. However, the performance of the imaging systems is inevitably degraded by the high-order diffractive light from the diffractive system. To address this problem, in this paper we mathematically deduce the imaging model, including multiple-order diffraction, by the scalar diffraction theory. After the imaging characteristics analysis, an adaptive Wiener filtering algorithm based on the principal component analysis method is proposed. The broadband imaging and deconvolution experiments are performed by the 80 mm diffractive optical telescope systems. Results demonstrate that this method increases the average gradient by at least 8.2 times and improves imaging quality and contrast. It could be a useful exploration for high-performance imaging of large-aperture and lightweight telescopes.