The life cycle of ferns involves a small, haploid gametophyte alternating with a large, independent, diploid sporophyte. Since spore germination and free-living gametophyte development are completely independent of the diploid sporophytic plant, the gametophyte sex differentiation is affected by various environmental factors. Factors studied in recent years include hormones (such as antheridiogen, gibberellic acids (GAs), abscisic acid (ABA) and ethylene), light, population density, nutrition (such as sugar and methionine) and metal ions (such as calcium and cobalt ions) to the fern gametophyte sex differentiation. Results of these physiological researches suggested that: (1) Antheridiogens induce male development and repress female development of the gametophyte, and GAs play a similar function with antheridiogens in sex determination. However, ABA blocks antheridiogen-induced antheridium formation. 1-aminocylopropane-1-carboxylic acid (ACC), the ethylene precursor, can enhance the number of antheridia induced by GA, while an inhibitor of ACC synthesis aminooxyacetic acid (AOA) restrains antheridia formation via inhibition of cell division. (2) Light intensity and quality affect sex expression, while the effect is varies by species. (3) Sugar addition not only promotes male sex expression in young differentiating Equisetum gametophytes, but also accelerates the onset of hermaphroditism in mature females. (4) Calcium is required in blue-light-promoted and red-light-inhibited antheridiogenesis in the fern Anemia phyllitidis. Moreover, increasing methionine concentrations mixed with GA3 enhances the number of precociously formed antheridia, and cobalt ions also stimulate the GA3-induced antheridia formation. (5) Population density also affects gametophyte growth and sexual expression. Female and asexual gametophytes dominated in populations of low and high densities respectively while hermaphroditic and male gametophytes were dominant at intermediate population densities. Furthermore, recent mutant phenotype analysis and molecular biological research revealed that some specific genes, including ANI1, ANI1, HER, TRA, FEM, and MAN, involved in the sex differentiation under the regulation of antheridiogen. This paper gives an overview of ferns sex determination in response to environmental factors and its mechanism.