Since the breakthrough by Tang et al. in 1987, organic light-emitting devices (OLEDs) have attracted extensive attention in the industries and academic research communities. OLEDs have many promising characteristics, such as self-illumination, lower power consumption, easy fabrication and so on. It has a broad development prospect in high resolution display and other fields. For RGB color OLED display technology, blue light organic material is very important. Polyfluorene (PFO) is a kind of rigid planar biphenyl structure compound in all kinds of OLEDs blue light materials. However, PFO has a very big disadvantage: the long wave shift of the light-emitting peak of the electroluminescent device will produce the green light-emitting band that should not have appeared. This seriously affects the saturation color purity of PFO devices, and also seriously restricts the industrialization process. In this paper, the molecular magnetic material Fe(NH₂trz)₃· (BF₄)₂ is used to solve this problem. ITO/PEDOT:PSS (30 nm)/PFO:Fe(NH₂trz)₃·(BF₄)₂ (65 nm)/CsCl (0.6 nm)/Al (120 nm) devices were fabricated on ITO glass substrate. It is the first time to report the strong pure blue emission of PFO by using the special electronic spin state modulation of Fe(NH₂trz)₃·(BF₄)₂. The influence of Fe(NH₂trz)₃·(BF₄)₂ on the photoelectric properties of PFO was studied in detail by analyzing the PL and EL characteristics of PFO and PFO:Fe(NH₂trz)₃·(BF₄)₂. Under the bias voltage of 4 V to 9 V, the device without doping Fe(NH₂trz)₃·(BF₄)₂ emits very strong green light. The central peak wavelength is 553 nm, and the color coordinates are (0.33, 0.45). Moreover, with the constant change of voltage, the green light-emitting band is always much larger than the blue light-emitting band. However, the obvious difference is that Fe(NH₂trz)₃·(BF₄)₂ doped device emits strong blue light, the peak wavelength is 438 nm, and the color coordinates (0.23, 0.22), which is completely consistent with the peak wavelength of the PL spectrum of the PFO film; the green light-emitting band of the PFO is successfully suppressed; with the change of the electric voltage, the proportion of the blue light part of the device spectrum in the whole EL spectrum is almost unchanged. The photoconductivity effect of undoped Fe(NH₂trz)₃·(BF₄)₂ device is further studied by means of the integrated opto-electro-magnetic measurement technology. Under different bias voltage, it is found that there is almost no excimer in PFO:Fe(NH₂trz)₃·(BF₄)₂. This study solves the problem of green light of polyfluorene, which has puzzled the industry for many years, and provides a reliable way for the industrialization of polyfluorene used in blue OLED. The mechanism of Fe(NH₂trz)₃·(BF₄)₂ blocking the abnormal green emission of PFO was discussed by using the theory of luminescence dynamics.