Sun-induced chlorophyll Fluorescence signal（ Fs） is related to photosynthesis and can serve as a direct indicator to monitor plant photosynthesis status. Fs is retrieved using the three most common Fraunhofer Line Depth（ FLD） retrieval methods

namely

original FLD method（ s FLD）

modified FLD（ 3FLD）

and improved FLD（ i FLD）. These methods exploit spectrally narrow atmospheric oxygen absorption bands and relate Fs to the difference in absorption feature depth between fluorescensing and non-fluorescensing surfaces. However

owing to the nature of these narrow bands

Fs retrieval results depend not only on vegetation species type or environmental conditions

but also on instrument technology and processing algorithms. Thus

many uncertainties remain in different Fs retrieval algorithms that use the two oxygen absorption Fraunhofer lines at 688 nm and 760 nm. This research clarified the uncertainties in different Fs retrieval algorithms that use the two oxygen absorption Fraunhofer lines to optimize the remote sensing detection index of chlorophyll fluorescence and improve the inversion accuracy of chlorophyll fluorescence.This study employed the Fluor MOD model to simulate canopy spectra under different chlorophyll contents

Spectral Resolutions（ SRs）

and Signal-to-Noise Ratios（ SNRs）. s FLD

3FLD

and i FLD algorithms were also used to retrieve chlorophyll fluorescence. The Fs retrieval accuracies of these three popular algorithms were investigated under different chlorophyll contents

SRs

and SNRs using the simulated spectral data by Fluor MOD model.Results are as below.（ 1） All the three algorithms have higher precision in the O2-A band than in the O2-B band.（ 2） In general

the s FLDs method strongly overestimates Fs

whereas 3FLD and i FLD provide an accurate estimation of Fs.（ 3） In the O2-B band

i FLD method performs best when chlorophyll content is 10—40 μg / cm2

3FLD method performs best when chlorophyll content is 40—70 μg / cm2

and the s FLDs method performs Verhoef best when chlorophyll content is 70—80 μg/cm2. In the O2-A band

3FLD method always performs best in any value of chlorophyll content.（ 4） SR and SNR specifications would introduce a noticeable error for retrieved Fs. SR is the dominant factor for s FLD method

whereas SNR is the dominant factor for i FLD method.In conclusion

the three algorithms have their own limitations and advantages under different parameters. Fs retrieval error results from the estimation error of the ratios of reflectance and Fs inside and outside of Fraunhofer lines

in which chlorophyll content is the most important key variable affecting the three Fs retrieval methods. Sensor performance also has a significant effect on fluorescence extraction results. Technical sensor specifications and retrieval methods cause significant variability in retrieved Fs signals. Results are intended to be one relevant component of the total uncertainty budget of Fs retrieval and must be considered in the interpretation of retrieved Fs signals.