Beam wander is an important effect when a beam propagates in atmospheric turbulence. Based on the marine atmospheric refractive index fluctuation power spectrum, a new theoretical expression is developed for a horizontal propagation path in maritime atmospheric environment. In order to validate the model, a set of measurements of beam wander for a collimated Gaussian beam is carried out using CCD technique during multiple laser transmissions under the sea surface environment in Yantai region, and the experiments are conducted at different times and various ranges. By comparing the theoretical value with measured value under the weak fluctuation condition, the results show that the relative errors between them are below16%, and the estimation given by the new theoretical expression generally provides a good fit with the data. According to the conclusion of this paper, the beam wander in the maritime atmospheric turbulence can be more accurately predicted.