Although the neural basis underlying visuospatial reasoning has been widely explored by neuroimaging techniques, the brain activation patterns during naturalistic visuospatial reasoning such as tangram remains unclear. In this study, the directional functional connectivity of fronto-parietal networks during the tangram task was carefully inspected by using combined functional near-infrared spectroscopy (fNIRS) and conditional Granger causality analysis (GCA). Meanwhile, the causal networks during the traditional spatial reasoning task were also characterized to exhibit the differences with those during the tangram task. We discovered that the tangram task in a natural environment showed enhanced activation in the fronto-parietal regions, particularly the frontal cortex. In addition, a strong directional connectivity from the right prefrontal cortex to left angular gyrus was detected for the complex spatial reasoning condition of spatial reasoning task, whereas no effective connectivity was identified between the frontal and parietal cortices during the tangram task. Further correlation analyses showed that the behavioral performance in the spatial reasoning rather than the tangram task manifested the relationship with the connectivity between the frontal and parietal cortex. Our findings demonstrate that the tangram task measures a different aspect of the visuospatial reasoning ability which requires more trial-and-error strategies and creative thinking rather than inductive reasoning. In particular, the frontal cortex is mostly involved in tangram puzzle-solving, whereas the interaction between frontal and parietal cortices might be disrupted by the hands-on experience during the tangram task. Our study also indicates that conditional GCA combined with fNIRS neuroimaging technique is a robust tool for constructing the causal networks associated with natural visuospatial reasoning, which paves a new avenue for an improved understanding of the neural mechanism underlying tangram.