Novel colorimetric and ratiometric fluorometric dual-mode N, P-co-doped carbon nanodots, BPEI-CDs, for highly sensitive and selective detection of formaldehyde (FA) were successfully prepared from N-(phosphonomethyl)iminodiacetic acid (PMIDA) and branched polyethyleneimine (BPEI). The treatment of FA caused a remarkable linear enhancement of ratiometric fluorescence (F/F) in a wide range of 0-40 μM with a detection limit (LOD) of 0.47 μM (3σ/k), along with distinct color changes from colorless to light yellow. Mechanistic study shows that this electron-rich system, formed by the cooperative roles of N and P, promoted the FA-induced Schiff bases formation reaction, which contributed to the CD aggregation-induced emission (AIE) "turn-on" response and enhancement of π-conjugation-induced bathochromic behaviors. Furthermore, N, P-co-doped BPEI-CDs were successfully applied to the determination of FA in bean sprout samples. Using the standard addition method, the recoveries ranged from 96.9 to 101.8%, and the relative standard deviation (RSD) was in the range 2.23 to 3.21%. The application for intracellular FA sensing further verified that this novel nanoprobe may offer a new venue for the design of simple, low-cost, and sensitive biosensors. Graphical abstract.