We present a novel encryption method for multiple images in a discrete multiple-parameter fractional Fourier transform scheme, using complex encoding, theta modulation and spectral fusion. All pairs of original images are encoded separately into a complex signal. The spectrum of each complex signal can then be scattered into various positions in the spectral plane and multiplexed into one spectral image with a combination of theta modulation and spectral fusion. After Fourier transforming back to the spatial domain, the multiplexed signal is encrypted in the discrete multiple-parameter fractional Fourier transform domain. Information about the original images can only be successfully decrypted given the possession of all correct keys. The parameters of chaotic pixel scrambling for the proposed method enlarge the key space. Moreover, the proposed method solves the crosstalk problem of multiple images and improves the multiplexing capacity. Numerical simulations demonstrate the effectiveness of the proposed method.