The objective was to explore the relationship between the levels of serum and urinary free light chains (FLCs) during the progression of renal damage in multiple myeloma (MM) patients. We examined 91 cases of MM patients, detected levels of serum FLCs (sFLCs), urinary FLCs (uFLCs), and serum creatinine at the same time, and then compared sFLC and uFLC levels during normal and abnormal serum creatinine phases. Among the 91 MM patients, 22 patients had abnormal serum creatinine levels (no uremia), and 69 patients had normal serum creatinine levels. The levels of sFLCs and uFLCs in patients with abnormal serum creatinine were beyond normal, namely both serum and urine positive (serum+ and urine+), and the average concentrations of kappa FLCs and lambda FLCs were 516.76 and 604.67 mg/L, respectively. Of the 69 patients with normal creatinine levels, there were 39 and 30 cases of kappa-type and lambda-type MM, respectively. Of the kappa-type patients, 11 cases were serum positive and urine negative (serum+ and urine-) with an average concentration of 55.47 mg/L, and 28 cases were serum positive and urine positive (serum+ and urine+) with an average concentration of 513.09 mg/L. Of the lambda-type patients, 16 cases were serum positive and urine negative (serum+ and urine-) with an average concentration of 78.44 mg/L, and 14 cases were serum positive and urine positive (serum+ and urine+) with an average concentration of 518.08 mg/L. The levels of uFLCs did not parallel those of sFLCs. In addition to sFLC levels, renal function affected uFLC concentrations. As MM progressed, the concentration of sFLCs increased in a step-by-step manner, and the uFLCs changed from negative to positive to negative again. Therefore, the whole progression included three phases: sserum+ and urine-, serum+ and urine+, and then serum+ and urine-.