With the improvement of consumption level, the internal quality of fruit has become an important factor to attract consumers. However, the traditional methods used to measure soluble solids content (SSC) and firmness are destructive. To realize rapid non-destructive detection on the internal quality of multi-fruits, a handheld non-destructive detector for internal quality of multi-fruits with replaceable probe was developed. The hardware system of the detector consisted of a host and a multi-spectral acquisition probe. The host included a microprocessor, a power management module, a voltage regulator drive module and an input and output module. The multi-spectral acquisition probe included 12 light emitting diodes (LEDs) at different wavelengths and a digital optoelectronic sensor. The software system of the detector was developed in C language in the development environment at MDK 5.0. The diffuse reflectance multi-spectral of “Huayou” kiwifruit and “Xue” pear were collected by the developed detector, and the prediction models for SSC and firmness were established based on the partial-least-squares regression. After downloading the model into the detector, the detection performance of the detector was tested. The results showed that the root mean square errors of SSC and firmness prediction for kiwifruit were 1.51% and 5.13N, and the root mean square errors of SSC and firmness prediction for pear were 0.52% and 4.57N. Moreover, the measurement could be realized in 2s. The measurement on the internal quality of multifruits was realized by replacing the probe of the detector.