Abstract: Harmful algal bloom (HAB, also termed red tide) has increasingly caused tremendous damage to fisheries worldwide. Since the formation process of HAB is still to be uncovered and the causes of HAB occurrence are largely unknown, it is impossible to take effective measures of prevention. At the present, the only viable measure against HAB is to forewarn and predict the occurrence of large scale HAB, which relies on a viable and efficient indexing method. Unfortunately, there is currently no reliable method to forewarn the occurrence of HAB. The HaiDex method is of a diffusion-characterized water pollution indexing technology, which is invented to effectively forecast HAB, independent of water regions around the world. To ensure forecast accuracy, the HaiDex method is invented (and claimed) to: 1) Characterize statistically a continuous formation process with imperfect panel data of water quality (e.g.