<p>Nonsuicidal self-injury (NSSI) is an increasingly serious public health problem among adolescents, but little is known about the short-term concurrent and temporal associations among emotions, cognition and NSSI in daily life. This study employed the Ecological Momentary Assessment (EMA) method to investigate the within and between person associations among negative affect, rumination, and NSSI thoughts and behaviors among adolescents in the natural environment. Thirty-five Chinese adolescents (aged 12–19 years; 77% female) receiving psychiatric outpatient care completed EMA surveys three times daily for 14 days, yielding 1,129 observations. Bayesian dynamic multilevel models were used to estimate concurrent and lagged associations at both the within- and between-person levels. At the within-person level, increases in negative emotions and rumination were associated with increases in concurrent NSSI thoughts and behaviors. The negative emotions in the previous assessment were associated with subsequent NSSI thoughts and behaviors, while rumination did not show a lag effect. At the between-person level, higher average levels of negative emotions were associated with a greater risk of NSSI thoughts. Higher average levels of rumination were associated with more NSSI behaviors. These findings highlight that negative affect may serve as short-term risk marker of NSSI in daily life, whereas rumination primarily reflects a stable vulnerability rather than a short-term temporal driver.</p>