Background Pressurized metered-dose inhalers (pMDIs) used with spacers are considered the method of choice for delivery of inhaled drugs in preschool-age children. The aim of this study was to determine the effects of modifying the visual inhaler spacer usage guidelines on the correct usage rate. Methods The parents and caregivers of patients <6 years old who were prescribed inhalers with spacers for the first time were included in our study. The participants were randomly divided into a modified visual inhaler spacer usage guidelines group and an unmodified visual inhaler spacer usage guidelines group. All study participants underwent face-to-face interviews and completed questionnaires. Results A total of 510 participants with a median age of 31 (range, 20-46) years were included in this study. The modified visual guidelines group included 254 (49.6%) participants, and the unmodified visual guidelines group included 256 (50.4%) participants. One hundred sixty-five (65.2%) of the 254 participants in the modified visual guidelines group correctly demonstrated the inhaler spacer technique. In contrast, only 21 (8.2%) of the 256 participants in the unmodified visual guidelines group correctly demonstrated the inhaler spacer technique (p < 0.001). When comparing the inhaler spacer usage steps between the 2 groups, the modified visual guidelines group demonstrated the steps more correctly and more quickly (p < 0.001). Conclusion The current visual inhaler spacer usage guidelines are insufficient. We believe that improving the visual inhaler spacer usage guidelines, in particular, will increase the correct usage rate and decrease the number of usage errors.