Abstract: This study presents a systematic investigation of the complex radiation behavior of the long-period pulsar J1840−0840 based on dual-frequency observations conducted with the Giant Metrewave Radio Telescope (GMRT) in India and the 100-meter Effelsberg radio telescope in Germany. The average pulse profile of this pulsar exhibits two distinct and spatially separated radiation components. The frequency evolution of the average profile is consistent with the theoretical predictions of the classical radius-frequency relation, indicating that the radio emission height of the pulsar systematically decreases with increasing observation frequency. Single-pulse analysis reveals significant pulse nulling and subpulse drifting characteristics in this pulsar. The nulling fractions at 625 MHz and 1358 MHz are 32.63% \pm 10.46% and 28.36% \pm 7.35%, respectively. The occurrence rates of both pulse burst and nulling states exhibit an exponential decay with increasing duration. Additionally, PSR J1840−0840 shows quasi-periodic nulling behavior, with two modulation periods at 625 MHz, namely (50.57 \pm 9.03) and (21.33 \pm 3.91) times the spin period, and one modulation period at 1358 MHz, which is (31.37 \pm 11.90) times the spin period. By modeling each drifting band, the phase separation, drift period, and drift rate evolution over time were obtained. The results indicate a significant frequency dependence of the subpulse drifting parameters.