The method of forest biomass estimation based on a relationship between the volume and biomass has been applied conventionally for estimating stand above-and below-ground biomass (SABB, t ha −1 ) from mean growing stock volume (m 3 ha −1 ). However, few studies have reported on the diagnosis of the volume-SABB equations fitted using field data. This paper addresses how to (i) check parameters of the volume-SABB equations, and (ii) reduce the bias while building these equations. In our analysis, all equations were applied based on the measurements of plots (biomass or volume per hectare) rather than individual trees. The volume-SABB equation is re-expressed by two Parametric Equations (PEs) for separating regressions. Stem biomass is an intermediate variable (parametric variable) in the PEs, of which one is established by regressing the relationship between stem biomass and volume, and the other is created by regressing the allometric relationship of stem biomass and SABB. A graphical analysis of the PEs proposes a concept of "restricted zone," which helps to diagnose parameters of the volume-SABB equations in regression analyses of field data. The sampling simulations were performed using pseudo data (artificially generated in order to test a model) for the model test. Both analyses of the regression and simulation demonstrate that the wood density impacts the parameters more than the allometric relationship does. This paper presents an applicable method for testing the field data using reasonable wood densities, restricting the error in field data processing based on limited field plots, and achieving a better understanding of the uncertainty in building those equations.cannot be conducted directly [3,4] due to restrictions like heavy load of fieldwork [5,6], non-destructive measurement requirements [7], and difficulty of belowground biomass (BGB) measurement [8][9][10]. Indirect methods have been applied in estimating forest biomass through the amount of growing volume [11]. These indirect methods can be classified [12][13][14][15] as three conceptually different types: (i) the empirical statistical approach, (ii) the biogeochemical-mechanistic simulation approach, and (iii) the remote sensing approach. The first type is conventional. It usually estimates the biomass using a biomass expansion factor (BEF) or biomass conversion and expansion factor (BCEF) [11], and biomass allometric equations [14,16]. The factors are helpful for converting the biomass conveniently, and can be improved by addressing the variation of BEFs over time [17,18]. For more accurate estimation, the volume-based biomass equations were also frequently employed based on detailed data of plots on each stratum. These allometric models are constructed based on measurements from field samples. Depending on the sample size, a number of tree-level and stand-level models have been developed for applications corresponding to different data sources. Recently, Di Cosmo et al. [16] have deeply discussed the characters, features, and uses of different...