Precision agriculture has the potential to improve crop yield and resource use efficiency while protecting the environment. Effective monitoring of crop biomass during the growing season is important for making in-season management decisions. One commonly used active crop canopy sensor for non-destructive estimation of crop biomass is the GreenSeeker sensor. The GreenSeeker normalized difference vegetation index (NDVI) can become saturated under medium to high biomass conditions. The Crop Circle ACS-430 is a new active canopy sensor with three spectral bands (red, red edge and NIR), and previous studies have indicated red edge-based vegetation indices have the potential to overcome the saturation problem of NDVI. So far, studies comparing these two sensor systems for estimation of spring maize biomass have been very limited. Therefore, the objective of this study was to evaluate the potential of the Crop Circle ACS-430 sensor to improve in-season estimation of spring maize biomass at different growth stages. A field experiment involving different N rates and planting densities was conducted in 2015 in Lishu County, Jilin Province in Northeast China. The GreenSeeker and Crop Circle ACS-430 sensors were used to collect spring maize canopy reflectance data at three growth stages (V5-6, V8-9, and V12-13). Plant samples were collected after sensor data measurements and aboveground biomass was determined. The results indicated that NDVI and the ratio vegetation index (RVI) of both GreenSeeker and Crop Circle ACS-430 sensorsexplained 89-92% variability in aboveground biomass across three growth stages. No obvious saturation effect was found with RVI compared with NDVI for both sensors. At the V5-V6 stages with low plant height, the vegetation indices (NDVI and RVI) of both sensors performed similarly. From V8 toV13 stage with aboveground biomass and plant height increased, the vegetation indices (NDVI and RVI) calculated from Crop Circle sensor explained 7-24% more variability in aboveground biomass than vegetation indices (NDVI and RVI) obtained with GreenSeeker sensor. The light intensity of GreenSeeker sensor decreases with measuring distance from the crop canopy. The Crop Circle sensor performance is not affected by measurement height at the range of 0.25 to 2 m above crop canopy. Based on these results, both GreenSeeker and Crop Circle ACS-430 sensors can be used to estimate aboveground biomass of spring maize at V5-V6 stages, while the Crop Circle ACS-430 sensor is more suitable for stages V8-V13.