Underwater laser communication has the advantages of high information transmission rate, low delay, and high confidentiality, making it gradually become a research hot spot. Turbulence effects are usually studied based on the turbulent refractive index power spectrum model. The power spectrum inversion method for oceanic turbulence phase screen simulations lacks a model for the effect of outer scale on optical properties, so that the recently proposed new models for the refractive index power spectrum of ocean turbulence that include outer scale have all been made single-parameter by simplifying the two-parameter exponent. However, the advantage of this simplification is not absolute in the calculation of optical parameters, in particular, the Nikishov power spectrum model in the form of a linear combination of scalar spectra is still used in most studies on the transmission theory in ocean turbulence. Therefore, in this paper, we first analyzed the necessity of the introduction of the outer scale turbulent parameter, and then proposed a modified Nikishov power spectrum model of oceanic turbulence containing outer scale parameters based on the Nikishov power spectrum model, and verified the model on the convergence characteristics of the power spectrum and the normalized spectrum. By comparing with the existing power spectrum models containing outer scale parameters, the proposed modified spectral form is simpler and easier to separate the variables than the existing power spectral models containing outer scale parameter, and can be used in subsequent studies to derive analytic theoretical expressions for optical parameters based on fluctuating optics theory, with the help of hypergeometric functions, to form a complete system description of the exponential spectrum. Considering the joint effects of seawater absorption, scattering and turbulence, based on the proposed power spectrum model, a phase screen simulation model of the composite seawater channel is established, and the influence of the outer scale on the probability density function of the received light intensity is studied under the exponential power spectrum model using the Monte Carlo simulation method based on the phase screen. The effects of outer scale on the optical properties of Gaussian beams and the effects of outer scale on the time-domain expansion of signals are analyzed by fluctuation theory and phase screen simulations. The proposed modified spectral model of the exponential Nikishov spectrum is verified, and a Monte Carlo simulation model of the phase screen of the composite seawater channel is established based on this model. The variation of optical properties with transmission distance and turbulent outer scale is investigated by phase screen simulation, and the correctness of the phase screen method to simulate the effect of outer scale on optical properties is verified by comparison with the defining equation calculation. The variation of turbulent time-domain expansion properties with transmission distance and outer scale is also innovatively investigated. The results show that the effect of turbulence effect on optical properties increases with the increase of transmission distance, which is reflected in the beam drift, beam diffusion and scintillation index properties; the larger the outer scale, the greater the effect of turbulence effect on beam drift, beam diffusion and scintillation index properties, the greatest effect is also found at infinite outer scale. The probability density distribution and the time-domain expansion characteristics of the optical intensity are less affected by the turbulent outer scale and are mainly affected by the transmission distance, and the time-domain expansion value shows an approximate quadratic function with the transmission distance. The results of this paper will contribute to an in-depth understanding of oceanic turbulence and provide a theoretical basis for further analysis of Gaussian beam propagation in seawater and the effect of turbulence on signal transmission characteristics.