The Inertial Navigation System (INS) is widely used for Autonomous Underwater Vehicles (AUVs) navigation with excellent short-term precision. However, the positioning error of INS accumulates along with time. It is imperative to appeal to other approaches to mitigate drift errors, especially for long-term sailing tasks. To that end, underwater Terrain Based Navigation (TBN) is effective in bounding the drift errors. In particular, the particle filtering (PF) method is extensively employed in TBN to tackle the highly nonlinear measurement equation. In our previous work, the statistical properties of the Digital Terrain Model (DTM) gradient were exploited, and we proposed to use three probability density functions (PDFs) to characterize the normalized data, i.e., Gaussian, Gamma and Weibull distributions. In this paper, the likelihood was modified according to the gradient fitting results, i.e., an optimal distribution selection. Moreover, to prevent the measurement data with large gradients from generating particles with lower weights, a pre-screen procedure was proposed to stabilize PF sampling. As shown and demonstrated in simulations, our proposed improved PF method outperforms the other comparative ones in terms of Root Mean Square Error (RMSE) and stability, as well as accuracy, especially for flat terrain data. In terms of computation cost, employing smaller amount of measurement data, the proposed method is faster than the standard PF method.